id drain out that gear oil and refill it with new stuff, if it was low and you rode it hard most likely there's gonna be metal shavings (more like a fine powder) and you don't want to just fill up since it'll circulate into the oil (ie, contamination is still there, adding oil won't really help)
id double-check everything on the bike (air filter, bearings, frame, and fork bushings, brake pad wear, etc etc) Since it sounds like the previous owner either didn't know how to maintain it or just didn't give a crap.
also, make sure its properly mixed, id dump out the fuel mix in there and run whatever the owner's manual says (usually its either 32:1 or 40:1)
what most likely happened was a "soft seize" since it was running super lean (due to the sparkplug being loose, and other factors) which caused it to die, most likely its gonna be scored, however, if you can get it to run, id just run it till it seizes good (then buy a top-end kit)
this forum is for homemade motorized bikes (basically bicycles with a motor), However, we are happy to help, BUT id recommend thumpertalk.com or a dirtbike forum for better information.