was out on pk50xl(110 kit) this afternoon,trying to clock up some miles towards running it in,been out on it before with no problems but today back wheel locked solid and engine died whilst riding,pushed it over to side of road and smoke was bellowing out from engine compartment,first thought was engine had seized,have been using a little extra oil while running in, after a few minutes tried kicking it up and it went?? got me home and has been fine since,the only thing i can think is it was either engine seizure or the clutch was so badly adjusted that it was never fully released and was being ridden the whole time??could this account for the temporary seizure?could it have been the clutch that overheated? any ideas gratefully recieved thanks
what you describe is a sieze… probably not a really bad one as it free’d up and ran. things expand when they get hot… try a hotter plug, less advance, richer jetting, or check your oil seals. listen for detonation/pre-ignition/pinking, it sounds like a tin full of nails being rattled, coming from your exhaust/motor. thats your warning that its overheating… you’ll get used to riding with your hand over the clutch 
If the clutch has been adjusted badly, gear changing would be difficult, and sometimes the bike creeps a little when idling in first gear, but that wouldn’t cause the engine to stop dead solid, unless it exploded inside the crankcase and jammed the gears. In my experience, your engine seized. It maybe because of too much advance in ignition timing causing your engine to overheat. Sparkplug heat range incorrect is another cause of overheating. Another is your oil pump or oil metering device is defective or not „primed“ during reassembly.