As long as you build your vehicle with all new parts, and have receipts to match; then you are allowed one reconditioned part, which is normally the engine for most people (and also then allows people to go for smoke test only if you have age related block). If you do this then you get a new registration, Simples
Those are the rules, which are well documented if you care to subscribe to the church of google and spend some time researching... This took me a massive 15secs...
Registering a kit car or kit conversion : Directgov - Motoring