People have swapped in 2.4, 3.1, even shoehorned a 3.8 and a 350 in there. There are even people nutty enough to ponder putting an RX7 rotary engine in as well. It all depends on how much skill you have and how fat your wallet is. Anything is possible if you have enough of the latter, even if you have little of the former.
Remember that if you upgrade to a "modern" engine, your going to have to bring along with it all the emmissions, electrical, and computer as well since much of it has signficantly changed since '91.
Check out the performance section of this forum and there are several threads similar to this topic already going.