As long as the block hasn't been windowed, it's most likely cheaper to rebuild it than to purchase a reman engine or longblock. Most I've seen listed at the parts houses carry a ridiculous price tag.
If you plan on keeping the car for a good while longer, rebuild the one that's in it. I'll bet I know what happened and it probably needs at least a crank, and probably a rod or two depending on how bad they beat themselves on the crank.
I think I had somewhere in the neighborhood of 400-500 dollars in rebuilding mine. I put a reman crank in it, had a valve job done, the cylinders honed, and the head and blocked checked for surface straightness. I had to replace one rod, I put in a new oil pump and screen (the reason mine blew in the first place), and all new gaskets. Kept the original pistons, the cylinders only needed honing for the new rings. Threw a new timing belt at it while I had it all apart.
The 2.3's are stout motors. They are quite capable of lasting longer than the car around them. The later models have a couple of problems that can lead to major parts breakage due to lack of oil pressure. If it weren't for that, they'd be just fine.
Anyway, if your Dad is considering getting rid of the car, snag a motor out of a junkyard ( for simplicity's sake you'll need to make sure it came out of a '91 and later Mustang or '90 and later Ranger) and throw it in there to get it running and slap the for sale sign on it.
Good luck in whatever y'all decide. We'll be here to help if you run into trouble.
Oh and welcome to Stangnet!!