A wide range of reducers suit P38s given the moderate power demand and lots of available under bonnet space... I've probably seen just about every type of sequential system reducer (and pairs of reducers in case of lower powered reducers) on P38s.
But I'd steer clear of reducers which don't have much range of pressure adjustment. Some otherwise decent reducers such as Zavoli Zeta S can be a problem if the narrow range of pressure adjustment doesn't suit the combination of engine and injectors... E.g. the Zavoli Zeta S really only likes to work at 1.5 bar, the older design of OMVL Dream reducers used to be widely pressure adjustable (between 0.8 bar and 1.8 bar) but with some messing about swapping out springs etc and some of the newer OMVL Dream reducers don't seem to like different pressure even if you swap internal springs. So even though a wide range or reducers can work on a P38 it's best to talk about those I'd advise given your injectors etc rather than just say anything goes... Magic3 high power, Romano HD, Emer Palladio should all fit the bill but they all have 8mm inlets, need external filter solenoids, use 16mm water hose and 12mm gas outlet.
I like the performance of the Bigas double reducer but it is bulky, heavy and is a pain regards plumbing. It really is just 2 reducers just stuck back to back sharing a common coolant channel so it has 2 pressure adjusters (1 for each reducer) and balancing their pressure output can be tricky.
The 2 gas outlets must be merged before feeding injectors to ensure both sets of injectors get the same pressure under all conditions...there'd be little chance of each set of injectors getting the same pressure over the engine's full load range if each of the reducers fed a separate set of injectors even if the reducers pressure was balanced properly at some points in the load range, the only condition where you get chance to balance reducer pressure is at idle unless on a rolling road. If you find your outlets are not merged (old installs would use a couple of Y connectors, one to merge, one to split back to separate feeds for the 2 sets of injectors) these days instead of using a couple of Y's you could just fit a H filter (2 inlets 2 outlets)
Even when the outputs are merged, if one of the reducers is set to say 0.9 bar and the other is set to say 1.5 bar the one set at 1.5bar will do almost all the work until it's flow limit is reached at which point pressure will quickly fall to 0.9 bar before the other reducer even starts to do any work, so in this situation you'd effectively have a reducer rated at only 190bhp instead of 380bhp because a system set to work at 1.5bar will probably be approaching the point of switching back to petrol due to low pressure at 0.9 bar. OK if you get the pressure settings near enough balanced, say within 20%, they really are good for 380bhp probably more like 430bhp. To balance pressure first make sure outlets are merged, engine running, watch pressure in LPG software, turn one of the reducer's anticlockwise (increasing pressure) a bit and see if it made any difference to pressure reading, if not try turning the other reducer's pressure up a bit... you're trying to find the one with already highest pressure.. then if you turn the other reducer's pressure up until it just changes the reading on screen the pressure outputs will be balanced.. but confirm the results a few times by turning the pressure screws both ways on both while watching readings. You can perform the balancing the other way around (looking for lowest pressure) but it doesn't work exactly in the opposite way because the pressure reading at idle will obviously always reflect the higher pressure, so you'd be turning down pressure on the one with highest pressure until when you lower pressure a bit more the pressure reading on screen doesn't drop any lower and then pressure should be matched... provided you didn't turn the pressure down on the one you just adjusted too much lol.
Dunno how bad your water leak is. If your reducer just has a stain of coolant on the outside, water doesn't leak into the gas outlets and gas doesn't pressurise your cooling system it might not be bad enough to change or rebuild, though a different model reducer might make for neater plumbing.
AEB ECU's such as your Bigas ECU flash the yellow light on the switch and temporarily switch back to petrol (without any beeps) if they've determined that 1. The necessary gas injector pulse length (given the calibration settings and any compensation for pressure and temperature) is longer than the available window for gas injector pulse length, while at the same time 2. Pressure reading isn't low enough that it would seem you've run out of gas.
If the switch beeps it's because the system has detected low enough pressure that it thinks you've run out of gas, then the switch back to petrol remains unless you press the switch to manually switch back to gas (obviously if you really have run out of gas it will very soon beep and switch back to petrol again).
V30 injectors come in a couple of flavours, some have internal holes only 2.5mm diameter, some have internal holes 3mm diameter. The nozzles they come with also have 2 flavours, some can be drilled out to 3mm, some not quite to 3mm. There would't be much point in drilling nozzles to wider than 2.5mm if internals are only 2.5mm because it would be the internals not the nozzles that would be the limiting factor in terms of flow, but it wouldn't be a problem because 3mm nozzles on injectors with 2.5mm internals would just flow as much as 2.5mm anyway. For 2.5mm nozzles I'd initially set the reducer output to around 1.2 bar, for 3mm nozzles 0.9 bar. The reducers I advised early in this post are all adjustable for that range of pressures while not compromising bhp capability but so too is your Bigas twin reducer.
FIxed a P38 this morning and advised the owner to become a member of this forum... He intends to and I'm sure he'll have plenty to chat about with you all (he has a leaky matrix and other problems)! On the phone he said he had an OMVL Dream system but it turned out he had a Dream reducer and injectors but Stag ECU. His car hadn't run properly on LPG since he'd had the reducer changed at another LPG firm. Don't know what spec Dream reducer he had before but this one was the HP (high pressure 1.7 bar) version. The combination of 1.7 bar and his OMVL injectors with 2.5mm outlets flowed too much gas for good calibration to be possible, for proper fuelling at idle the gas injectors would only need to be pulsed for 2.5ms which is less than the minimum pulse length for these injectors for accurate metering. He'd got a few OBD errors for no lambda signal on one bank and duff probe on the other but I got it set up great after adjusting pressure to 1.2 bar and doing a full recalibration while the lambda probes were working and it was running closed loop (perhaps temporary lambdas working - I advised him to fit new probes but the LPG system is calibrated great now). I was a bit surprised to be able to turn pressure down to 1.2 bar (from 1.7) on this newish Dream reducer without having to remove a spring and without rendering the reducer incapable of flowing enough gas for flat out, an old Dream reducer would have handled it no problem, this is why Dream reducers are not on the list of those I'd advise.
Sorry I haven't been around for a few days.