RE: two stroke or four stroke power?
four stroke will feel like it has a broader power band unlike two strokes which will feel like they have a very limited power band. In my opinion a saito 125 would be the best engine for it with a 16x6 apc prop on 30% nitro or a 15x7 apc prop on 15% nitro. If you go 2-stroke the 91 will not be enough for 3d, a 120 2-stroke would be the only other option and they are heavy.