I am going to agree with everyone saying that the rat piss did the damage. about a year and a half ago, I got an 85 250es that was siezed up. did everything you tried. I ended up taking off the cover on the left side that allows access to the 17mm bolt. Put a ratchet on it and got it to break free, then I worked it back and forth slowly. it was sized from rat piss...the airbox was full of nest, the metal filter cage was almost completely disintegrated, it looked as if it was at the bottom of the ocean for a hundred years. after I got it to turn over, it had zero compression. I tore the top end apart to find the rings and piston had become one from the rust. it might have been fine if the intake valve hadn't been open, but I had determined right away that it was, so I knew the engine was going to put up a fight.
Currently : trx400fw, ct70, 250es,200x, xl80s, 350x, 350x, 225DX, trx300