If your 50cc 4 cycle is good for 55 mph, the chances of improving significantly without getting loud or losing tractability are slim. Unless you want a race track only bike, enjoy it and buy a bigger one when you are able.
If you live anywhere near a motorcycle race track where they race small or old 4 cycles, go talk to those guys. Most of them will be only too happy to talk your ear off if you don't get in the way. You will learn more that you ever wanted to about tuning and what an involved process it is, but I am sure you will enjoy it.