Let me know if you can find a large brushless motor and controller for under $200 for that. It seems like the best way to get more performance from that cart would be to get some lipos to replace the lead acid battery. If you've seen Mike's lipo powered dirt bike, you'd know that the batteries really make a difference in the weight and power.
If you bought 4 of
these Turnigy 6s 5000mah 20c batteries and wired them in parallel, you would have about the same voltage, tons of punch, much less weight, and probably double the usable runtime. Then just bend the front steering assembly back into shape, mount some bigger tires on it, and you're good to go. Not sure how to make it 2wd in the rear, though.
Good luck!