My sincere advice is that you do not start by replacing properly-functioning parts. Tear the buggy down, freshen the diffs with new fluid, and get some new oil in the shocks. Replace anything that is bent or broken. Set up the car with the factory-recommended setup as it goes back together. Get the right tires on it. Then hit the track. Don't touch anything until you have specific concerns that you want to address. Make those changes one at a time, and verify that the expected results were achieved before going on to make other changes.