If you're not looking to build a hardcore SM, there are a few "cheap" modifications you can make. You can strap down the front suspension about 1.5 inches. When you're ready to do some off-road, just remove the strap. Renthal does make a number of styles of bars, that's what I'd suggest. As for the rear suspension, just leave it stock. ronny is right about the rims. You should be able to find a cheap set of rims from one of the smaller sportbikes. For tires, I recommend the Dunlop K-591. You'll get good longevity, and they grip fine for casual blasting.
If you need additional info, let me know. Have fun.