yea low quality hex keys can strip out. Had that happen on my 3d printer quite literally yesterday when I was trying to undo a seized grub screw. Found a decent one, and it came right out

That engine was probably stuck from the oil from the nitro fuel's residue. I had that happen on mine as they sat for over 10 years without being ran. I found em stuck as I was prepping them for sale, and making sure they still rotated before listing em(sold 2 out of 3. 3rd I'm keeping).

You should get small bottle of After Run Oil as WD-40 isn't really good for nitro. I think I paid like $2-3 for a bottle 15 years ago, and it still has like 95% in it as you only use like 2-3 drops after each run.

Also one question when you rotate the flywheel does the engine have more resistance at one point? I only ask as I'm not sure your level of expertise. As if it doesn't have and form or resistance, and feels the same it most likely would need a new sleeve and piston, but if it does there's a chance it might work fine.