I think all need done is re-register the car with the engine in, and insure it as is, giving the insurance company the reg, they'll look and see 1.8t you go yup, they don't need to know what past life the car has.
Thats like saying you have to declare if you have a bump into your driveway, have to replace the bumper, do it yourself, insurance dont need to know or anything like that.
If you understand what I mean.
Even write-offs, they get a V.I.C test and thats it, nothing about making sure that total wreck is sound etc, thats what M.O.T's are for.