You could put lots of money into it.
However your most cost effective way to get performance would be to sell this car. Take the money from the sale and put it towards a Mustang GT. You will spend less money in the long run and have a faster, more reliable car.
Don't waste your time with cat back systems, cold air intakes, or fancy spark plugs. Very little bang for the buck and little noticeable improvements.
Another way to go would be to add Nitrous
Oxide. However I would limit this to a 50 horse power shot. Nitrous can be completely safe if used correctly. It is only when people get carried away and try and get too much power from an older small engine that they get into trouble. A dry nitrous setup capable of giving 50 to 75 horse power will cost about $300 - $400. Not much more than a cold air intake that will only give you about 5 horse power.