you will still need to think about:- if it would fit in the frame, would you need mounts?, will the exist for the new enginee fit in the space?, and can you wire up the new engin? (probably need to make a new loom) plus you will need to notify the dvla about any changes to the bike ......then you need to get it to pass an mot, and last but not least you need to be able to insure it! all in all your looking at a mega amount of work! or about £3000 if you pay someone to do it for you.
ur best bet would be to just buy a new engine exactly the same as the old one then its just a case of whip old engine oit and pop new one in .....notify the dvla and away you go.