NEw stang engine

Anywhere from $100 and up. Depends on where you look, rebuilt locally, rebuilt from Autozone, or used, from a salvage yard, or from a private party deal.

I have one supposedly good 90 2.3 that I got for about the above price. And 2 more, that are not as good, but that came with whole cars attached for similar prices....

This motor is easily and commonly rebuilt. Most machine shops can handle it easily. Their biggest business is in the beginner circle track area.
 
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!!
 
Typical problems with the oiling system on the dual plug motors are stripped oil pump driveshafts and clogged oil pump pickups due to the oil pan o-ring gasket deteriorating.

Both cause them to lose oil pressure and then trouble comes a knockin. The clogged oil pump pickup sometimes shows up as slightly lower oil pressure and valvetrain clatter at idle. The oil pressure drops slowly over time giving the impression of a tired motor until the screen becomes completely clogged.

The weird thing about it is I don't think the problem lies in all of the motors. Some do it and some don't. I would imagine the early models are most suspect. My 93 2.3 Ranger never even coughed, let alone had oiling issues. So maybe they changed shafts and/or gasket material?