Hiya Alik,
Nice to have you aboard Sir,

Re the registering thing, I'd go with MSVAing it. That's what I plan with the missus' trike & my own new one. You hear lots of horror stories about the test -but they're all from people who haven't actually done it. ByzMax calls in here occasionally as well as the BritChopper forum. He's put several trikes through the test, most recently his VW "beach Bastard", (where's that write up you promised Byz?!). He says it's nowhere near as bad as you'd think -all common sense stuff, no pointy sticky out bits, no 300 decibel exhausts, all the lights in the required places etc. Worth investing in the MSVA manual.
Re finding a Reliant motor for your project. You could PM Dave Satan -he's got one for sale here at the moment.