We've established that Mechanic's loss in value is $8,000. Has he suffered any "other loss caused by the breach?" (Remember, the contract called for him to rebuild the engine for a price of $8,000. He did in fact rebuild the engine, and Enthusiast (the breacher) refused to pay.)

Yes. He has suffered the loss of whatever it cost him to rebuild the engine.

No. There was no "other loss caused by the breach."