Try heating it up and then use a standard hex driver or next size metric driver to loosen the enlarged bolt. The OS is known to take a good amount of lean run before seizing. You may have had little oil in your fuel. I used to use Wildcat in my BVM 91/96 which has 22% oil.

If the piston and liner are damaged, then change the bearing if they appear rusty or trun rough. The brg cost about $20 US. Towerhobbies has these engines for $280 or so, may be better to buy a new one!

A well treated OS 91 DF should last at least 100 flights. Run them as rich as you can for take off. I would also use pipe pressure if you can...Good luck!