For a water cooled motor, you really don't want to get into heavy engine mods, you may be able to bore it out +.10, but that's not giving you any real change in displacement. Keep the C50 stock, and look at maybe a C90 or 109. I prefer the 90 just because it keeps more of the same body style as the 50, just almost twice the displacement. This is exactly what I did, and while I loved my 50, I have no regrets upgrading.
More fun to ride it than wash it.