Julie Brooks-Girskiene I think I read about some company, maybe Jaguar I don't remember, taking back their old classic cars and converting them for customers, but they give you the original engine and bits so that it can be changed back in the future if you want to.
Aston Martin were/are doing this for owners of classic old models.
They swap out the conventional drivetrain for all electric, then refurbish your engine if needed, crate it up and return it to you. The entire process uses all original mountings, etc. and apparently doesn't do anything that can't be reversed to exactly as it was before, so that all of the original value of the car is preserved.
I think that Jaguar are, or are about to, so something very similar with classic E-type cars.