from what I've been told $7500 for a crate Boxster engine. They don't rebuild them, they just swap them. The spec Boxster series mandates stock engines, so opening them up isn't even an option.

Keep the 951 and buy a spec-miata for racing. Very cheap to run and probably faster lap times than your stock 951. I see fully preped- used ones in good condition for less than $10,000 all the time.

I drove a spec-miata at the track a few weeks ago. It was a lot of fun, but I hear you won't come away from a race without dented fenders. Since you're required to not to have dented fenders at the start of a weekend, I would imagine it could get kind of expensive too.