Look for loose/dirty/corroded wiring connections to the distributor. You'll need a wiring drawing for '91 if you want to know which wires are for the CKP sensor. But there aren't a ton of wires altogether, so you may as well check them all.

That CKP sensor is down in the bottom of the distributor, and I think it's not available by itself. If you check out the wiring, & rule out those problems, you'll have to buy a distributor.