Rockie - Check out this thread from another forum. He changed his final drives gears (and a bunch of other things), requiring a case split, and it hasn't worked out that well. its a long, detailed thread, but I think his biggest problem is that he changed too many things at one time, rather than making one change at the time and sorting out the bugs to determine the effectiveness.
I'm not telling you not to mod your existing scoot for speed. Set a budget and take your time in case a bigger engined scoot is the only thing that will reach your desired speed.
http://www.modernbuddy.com/forum/topic17276.html