The easy swap would be the V6 that came optional in them. The 3.1 is a fine fit, and the 3.3, 3.4, and 3.4 DOHC should fit without too much headache. But, the amount of work it would take to put in a different engine family would be insane.
Problem is, like most cars this age and size, the cost of the swap would be more than just buying a more powerful car, plus you'll have a vehicle with a better resale value so you'll get your investment back.
I know the urge to mod what you have is always strong, I have it to, but you have to remain logical. I say, save your pennies, drive this car for what it is: a nice economical car, then trade it on a Beretta GT with the 3.1 and a 5 speed. I had one for a long time and it was very fun.