Convert to 5 speed, plz -- some cars are OK with an automatic, but a Miata loses all of its charm with a f*cking slushbox.

However, if you just want to get the car running, why not replace the head gasket after checking the head for warpage and having it milled if needed? Not a hard job -- those cars are childishly simple to work on.

Once fixed, you could add a turbo if/when desired.