A P-reg cab could be any 1.8. Only the SI was 130bhp.

Even if it's a 130, I'd still consider it. The 130 engine isn't all that, would still feel notably quicker with the RS engine in, and has plenty of tuning scope.

It's best to swap the whole lot from a donor car; engine, box, shafts, hubs, loom etc etc, as it's all different, but you can fit it with the IB5 as said above. If it's a 130PS cab you're in luck, as the box and things is the same as the RS, so would be an engine and loom job.