You had the right idea, but maybe you took too big of a swing at fixing the problem. To get 50 MPH with 14/54 gearing, your engine had to be turning about 6200 RPM. With an 18/54 gearing and turning the same 6200 RPM would get you to 65 MPH. But that’s if you could still turn 6200 which you may not be able to do with the loss of the mechanical advantage of the first ratio you used. Going from the 14t driver to the 18t was equivalent to dropping twelve teeth on the axle. Try dropping a couple of teeth at a time on the axle gear and see what happens.
All of your speed does not come from the engine. Chassis setup and drive technique play a big roll in top end speed.