Me thinks something is going on here
8-|. What kind of oil are you using? Pre-mix or injection?
Did you EVER have a Fatty pipe on it? As to the spark plug, have you tried a search on here? As to the cold seize:
after doing a proper break-in (search) when you get it back from the shop, Start the quad and let it warm up with a few light blips of the throttle, until the cylinder is warm/hot to the touch, BEFORE running at full throttle.