Tuning the computer?

Well i just found out that the guy i bought my 95 GT had the computer "tuned" for a richer mixture so he could make some later mods (that he never did) to turn this car into a Saleen. This explains my Missfire at idle and the VERY black tail pipes. What im wondering is is there a way to tune the computer or does a person have to have a chip burned. Im assuming theres a burned chip in it. If thats the case were can i get a stock chip so i can get my mileage back until i have the money to do some mods to this car. Or is there a way to tune the computer with a scanner or PC? I know hypertech doesnt make a scanner for 95s. Would it be cheaper for me to just go ahead and make the mods (headers, exhaust etc) to lean out this rich mix?