Were there any numbers cast on the flat pad above the starter, behind the engine mount?
As for possible original parts, including block, in a 1991 car, parts can be dated before the year model of the car, but not later than.
You could hunt down the casting date on the block, and compare it to the build date of your car.
So long as the casting date of the block is before the build date by at least a couple weeks, you can narrow down if the block is wrong. You can never tell it is 'right' though, without the VIN. You can just narrow it down to likely, but not positive.