The cones in the subwoofer if my P38 Vogue have shredded. I'd post a picture but I can never get it to work.

It is an XQA100070.

They measure 3 ohms, so must be 4 with impedance...

But I'm clueless on power for speaker systems.

I think they are rated at about 50W. All you really need are some that will fit the hole.

Is it possible to rebuild the cones?

Yes, if you have access there are music shop specialists that can re-cone and put new membranes to your old speakers.
Have done this to a friend old Audi and another with a G-Klasse, both with unique speakers.
Although I have put new speakers on mine directly, just a matter of matching size and impedance.