No chance of a permit. The CAA won't allow it for factory builts these days.
An engine change is, in theory, allowable as a major mod, but it would have to be signed off by a CAA airworthiness inspector.
Probably a bit too major to be a viable proposition.
Why not buy/build a modern, Rotax powered, aircraft, or wait for the new Cessna LSA? It'd be a lot easier.