Craig I think you might be wrong about how easy it is to do it fwd vs rwd. Rwd would require a lot more cutting about, body strengthening, etc, not to mention the IVA requirement for road legality. Also you have to add a fuel tank somewhere. And thats assuming you can get away with reusing the cento rear trailing arms which I'd be doubtful of.
Fwd its a dvla paperwork exercise and you have fuel lines, wiring, and a nice suitable space to put the engine.
You *might* need custom driveshafts, although I got round that on mine by turning a 6 speed punto gearbox diff into the chain diff. Unbolt crown wheel and bolt on sprocket. All it needed was an adaptor plate which any decent laser cutter can do very cheap. By correctly positioning the diff laterally it allowed me to retain the cento driveshafts without modification.
The only real catch is that the engine ends up on the bumper due to sprocket sizes. IMO the only reason it shouldn't be done. A gear diff might sort that out but they're upwards of £600.