If someone were to buy a small engined car and then if said person were to do an engine conversion where a bigger engine was used what is the correct way to get this road legal?

What forms are needed and also is it any SIMI garage that can do the inspection or does it have to be a dealership?

Will dealerships stamp a form if they don't do the conversion? What do they check?

How if this is done is this declared regarding motor tax and also insurance?

I have seen a few threads on this but no real answers. Do most just go undeclared?