Well, you could always rebuild the engire from the ground up and take the time to shave down the heads a bit (there isn't much room, so be careful), and polish the ram air tubes, etc., and do some other misc performace things, but for the costs involved, I would reccomed dropping a larger engine in. I have an '89 Classic, and those originally came with 3.9 L engines. I am now running a 4.2 L Rangie engine. If you are looking at the cost involved, I wouldn't reccomend anything larger that a 4.0 ( I had to do extensive mods to my drive train, but I do all of my own work).
I know someone who supercharged his, but that was a lot of work, and it kept failing him when he took it wheeling, but it looked really cool and gave him a lot more power without sacrificing gas milage much- a big plus in these vehicles.
I hope this gives you an idea of what you can do!
Jeremy
