Hsukaria,
I also replied directly to your post as forwarded to my email. To repeat one or two things for the general audience:
Strip down the engine (remove piston, rod, sleeve.) Set rear face on a flat board. Shield the shaft end (prop nut a turn or two above shaft, block of wood as contact surface) then tap firmly on the prop end of the shaft. Once the collet loosens, it releases the driver shield.
Your fuel may be a problem. Is your fuel a commercial product, or your own making? What we "civilians" can buy isn't always of the quality or freshness that commercial fuel brewers get. And in humid, hot conditions fuel can absorb water from the air, or go off for other reasons.
20% castor is not ideal for iron in steel engines; 25% total oil should do better (you can make up the difference with synth.) I had some fuel ~25+ years ago in which the castor 'went bad' and visibly darkened in the liquid fuel. Suddenly! It formed crystalline, abrasive particles at the sleeve ports and made a scraping, screeching noise while spewing sparks! (No mufflers back then.) Seized hot after only several seconds... I fly CL, no throttle to shut it down on that one... The parts that suffered this were able to serve after cleanup!
Iron/steel engines CAN overheat and either feel about to seize or very low compression when they stop; I've seen it and done it in hot, humid mid-west summer conditions. They usually recover if allowed to cool several minutes to ambient temps. Proper castor% is the best defense.
Have you bench-checked the throttle setting? That lets you shut down instantly if things go wrong. After you cleaned it up, did you go through the throttle setting procedure recommended for the carb you're using? (I assume you checked that fuel passages and filters flow properly and if it is a flexible clunk tank, that the line is not crimped over...)
Again, luck!