That's going to be a new design job, almost certainly approved under LAA amateur design/build procedures to CS.VLA (or either BMAA or LAA as a Microlight to BCAR Section S, which may well be the easier route). Loads of information on both organisations' websites about that to work from, also some excellent textbooks: I'd start with Darrol Stinton's "Design of the Aeroplane".
G