You'll have to make sure the junkyard gives you 15 to 30 days to return the engine. 

Get that in writing. Be completely honest with your junkyard man, tell him which exact tests you'll be performing on the engine and which ones are a dealbreaker.

So first do a leak down test. That's a good general health of the engine test.

Also, smell the oil for gas. (No Good). Feel the oil for metal grit.(No Good)

Pull the spark plugs and inspect them.

I'd invest in a borescope. Then while you have the spark plugs out you can inspect the Pistons, cylinder walls, and check the valves to see if they're burnt but maybe not burnt enough yet to show up in the leak down test.

Also you can use it to look inside the valve cover through the oil fill hole at the springs and rockers and the valve train in general and also check for sludge. Maybe stick it down the dipstick tube also to look for sludge and obvious issues.

These days they're pretty cheap.

This Vevor costs around $100

Skip up to 3:52 in the video

https://youtu.be/tu0mNMlEC3U?si=lbXNiQxyRTqlHddE&t=232

@jack62 I got a klein ET16 boroscope for $40 that you hook to your smart phone. I love using it for things like this, and even checking my car frame for rust, in those places you can reach or see.

Those engines can run like a scaled ape if you take care of them. I have one in my 1999 Ranger 4x4 and it has almost 295k miles on it. All original engine. My transmission was rebuilt at 251k miles. It's my daily driver. 

The problem is, finding one that hasn't been scrapped due to the engine itself going out, either from overheating, blowing head gaskets, or oil starvation.

That engine is 25 years old and who knows what people have done to it. It ended up in the scapyard for a reason.Try to find a low mileage one whose rear frame has rotted out or wrecked.

The Ranger had two separate frames that were riveted together, and the rear frame was made of much lower quality steel than the front. Consumer groups advocated for a recall but that never happened. My Ranger's frame is starting to rot a little bit. Most of the damage is limited to a crossmember, I caught the frame rails when they were just starting to rot, and I had a friend who welds for a living weld some quarter inch steel plate to reinforce the frame after cleaning it up with a grinder. I hauled a couch, three tires, and a heavy chair to the dump a couple weeks ago. It's still okay.