As said before it WON'T be legal unless it is Mot'd, taxed & insured,
Look at the hassle this guy went through for a 25cc engine:
http://www.users.globalnet.co.uk/~pattle/nacc/arc0497.htm
My god. What a stupid country we live in.
What about these electric bikes I see all the old biddies on aroud here. Are they legal?