You will find the point where the petrol injector loom joins the lpg loom that you need to reconnect the wires to their original location. Provided you take some care thats not too difficult. If you don't do so, it will not run afterwards. It also will not run if you remove the LPG ecu and don't rejoin those wires.
Without seeing what you've got there, hard to say, but provided you only cut wires to the lpg system you should be ok, obviously you need to make sure they are away from any thing they could either get damaged by or short out against (so insulate them in some way if you can't do anything else).
You should find 16 wires going to the petrol negative injectors, a positive pickup from the battery (maybe your fusebox cable) another switched positive (maybe petrol injector positive) and rpm pickup (maybe). The rest of the cables should only run to the lpg bits (pressure sensor, vapouriser, tank wiring, control panel etc)
If your looking to sell it, keeping the loom as intact as you can would be preferable.