you can change the gearing if all you want is speed. Try going the the local hobby shop and picking up and larger pinion. This is the cheapest way to go and definately something to try first.
The only down side about gearing up, is you lose your torque at the bottom end. That is, your GT will start out slow, but will go fast once moving.
As for tuning the engine...you're going to have to ask the others for help on that. The instructions should give you the proper starting point and tell you how to do the final adjustments to get it running just right.
Why do you need to go 50mph???
If I can't break it, no one can...