Alas, $4500 is about right for a "new" engine, but 4K is way to low for 'parts'. The car isn't junk, far from it, so you depending on the year, you should see 10K from it if you want to go that route.
I'd persevere if I were you. Try fossiking the various MX5 sites, there are loads, and see whose got what in the way of recco'd engines.
IMHO its too nice a car to give up on. I wanted an MX5 or an MR2 because, with a bit of fettling they can be made zippy-quick and well fun!
But my wife wanted something "special', so I bought a 911C4. It scares the crap out of her and she won't drive it
.and I don't have the time!