I seem to remember that the 50mm was found to be adequate for flat land, and hills required more copper mass, so...there have been quite a few 63mm friction drives made in the past to find the limits of this style of drive. One benefit of the 63mm's were the lower factory Kv's that were readily available.
Experiments can be fun, but rather than repeat the invention of the wheel, I'd suggest you add two ripple-suppressing input capacitors. The 50V listed here seem to be the most popular, and well-proven...http://endless-sphere.com/forums/viewtopic.php?f=30&t=22194&start=25#p382837
