I found a 1990 Miata on Craigslist. He ask 350, we settled on 300. It wasn't running, had been sitting for apretty good while. Actually, I don't even know now whether it will run, but I figured I got a good deal. I'm using the wheels, complete rear sub, engine and trans, plus the ECU and a few other parts. I've already stripped it and got $63 from the crusher and sold a front strut for 30. I still have doors, rear deck lid and some glass, so I think I will actually make money on it and still have the parts on the locost car. Even if I have to buy another engine. You can buy a pretty good JDM engine for 6-8 hundred.
I found a strong running high mileage 1.8 liter on ebay for $350. It easily adapted to my 1990 Miata's ecu and what not. Just replaced the injectors and swapped over the 1.8 TB. No need to spend 6-8 hundy on a JDM engine. Those engines are found in everything from Proteges to Kia Sportages if memory serves so they are cheap and plentiful. They have piston oil squirters and such that are usually only utilized in turbo applications so don't fear high miles. Miata engines are rediculously bombproof once you get past the short nosed crank variety which is probably the problem with yours. Have a good close look at where the crank pulley meets the crank. I've seen them with the end of the crankshaft snapped right off, still loosely bolted to the pulley. In 1991.5 they went to a "long nosed crank" which mostly solved this problem. These are the engines you want.