Not too sure that your going to improve both very much without a whole lot of money invested. Horsepower can be improved significantly for around 500 and gas mileage can be inproved for around 1500.
Horsepower- cam and ignition upgrades
Gas mileage- Pop the top of it go to craigslist and buy a festiva. Take festive and throw it in back and when you run out of gas pull over unload festiva and finish errands.
Other than that I know nothing that helps with gas mileage sorry.