I have a 2005 Piaggio Typhoon 50 2T, completely stock, with 2277km on it. A year and a half ago in late fall I rode out of my garage, down the alley and turned on to the street where I gave it full throttle and it almost completely died. There was a horrible racket - it sounded like something let go inside the engine - and there was a sudden and significant power loss. It didn't die completely but continued to run roughly although it did not make enough power to move the bike. I shut it off and walked it back to the garage where its been parked ever since. I did try and start it a few times after but it was difficult to start and it ran very roughly.
I finally started tearing the top end apart expecting to find evidence of a catastrophic failure but the piston, rings, and connecting rod are intact, and the wrist pin and bearing look good. There is some evidence of blow-by but not too bad, the piston crown is uniformly covered in black deposits, the underside of the piston crown shows some black deposits also, the sides of the piston are smooth to the touch but there are faint marks that may indicate a soft seize. Cylinder walls are smooth to the touch with no visible damage, hone marks are still visible. Exhaust port has some buildup but does not appear significant.
I have quickly looked at the bottom end, everything turns smoothly, bearings do not appear to have any excessive play, but I have not taken it apart.
I'm fairly new to 2 strokes, having only rebuilt a chainsaw once. Does this sound like a soft seize? Would a new piston/rings fix it? Do I need to pull the bottom end apart? Anything else I should check?