Gear it up a bit by changing pulleys
weld the pivoting front axle solid for stability
weld rear end spider gears (if its a hydro find a new tranny)
atv tires for the back (may have to buy hubs and rims for the tranny)
cut the fenders out/make flares if needed
bigger front tires to level it out
reinforce/re-design steering for safety, stock normally is weak and sloppy.
re-mount seat as low as possible
remove anything unnecessary (deck mounts, lever for lifting/lowering, etc)
make yourself a gas pedal, and remove the governor on motor
Just a start, if u need help with anything let me know.
Heres a video of mine, best chassis's to use IMO are the older ones, must stronger. My wheel horse has a beefy *** tranny, with bearings all around and 3/8" thick cast iron cases, compared to brass bushing and 1/4" aluminum cases we see these days. Since the video it has a much better paint job with racing stripes and better tires.
http://www.youtube.com/watch?v=kMdE7pd0oqQ