50 might be a bit much for a Vegas as it was only designed for mid to upper 30s. As far as shaft upgrades, the general consensus has the flex cable upsized to a .187. That will let you use bigger props but, at the same time, it also means replacing the strut with one that a .187 will fit.
As far as engines go, the Nova Rossi 15 and K&B 18 are the only two I've found and, even then, I wouldn't trust MECOA, the manufacturer of K&B, to have any ready to ship, even though their website claims they do