you know what they say, "speed is only limited by budget..."
For several hundred $$, simple bolt-on mods can bring you from 160 hp to 180 horse at the engine: intake, exhaust, throttle body, maf, and chip. If you want more you will have to port and polish and possibly have some bore work done. If you want more than that you will have to go with a supercharger or drop a V8 of some sort in.
But just remember, there is only so much you can do with the OHV engine and a vehicle with brick aerodynamics and 4000+ lbs. The OHV engine was designed for torque and reliability, not for raw power.