MIP once made an exhaust replacement for the stock one on that vehicle. It was called the MIP stinger. I have it on my vintage nitro hawk. The kit came with a replacement shocktower because the exhaust went straight out the back and wouldn't fit thru the old tower. It gave the pro 15 a noticable power increase. I don't think they make them anymore, but you could probably find a used one on ebay.

For that motor, you could always buy a curved header and install a tuned pipe. I think associated makes tuned pipes for small block engines.