That's interesting Rufus. In the UK, as I understand it, it's the chassis that is registered and tested etc. Making your own chassis and getting approved is a regulatory nightmare, so if your building a car from the ground up borrow an already homologated chassis (see Top Gear when they build an electric car - the Hammerhead iThrust)
.......you can install any engine you like. A change to the reg docs is fairly straightforward and a notification to your insurer is pretty much it (I think!)