I'm looking at a medium Stanley chassis. It current runs asphalt but I'd like to have my son run dirt. Would I have to do allot to convert it to run dirt?

Just a few things , Tires, shocks n springs, gear ratios, maybe your right rear bearing carrier so that you can bring the right rear in to tighten the chassis . This helps hook the rear of the car on a dry slick track.
Good Luck and Have Fun