Suggest you remove that stud and see if the stud is stripped. This is not likely, as the case is aluminum and the stud is steel. You can go to a good local hardware store and get a Heli-coil, if it's a "80cc", chances are it's 8mm x 100 or x 125, bring the stud with you to the store so they can match it up. You will have to buy an appropiate size drill bit and tap, along with the helicoil to replace the threads. You will have to disassemble the top end of the engine, head and remove cyl. Stuff some soft rags around the piston(be careful) and take it to a machine shop or possible small engine repair/boat repair etc... they will have it done in short order. If by chance the stud is stripped, replace as needed, see side bar for stud kits. make sure the shavings are out of the case and you can try putting a extra washer on the nut too, just in case(not likely either) the stud keeps turning down. Don't forget to order a base gasket and possibly a head gasket(most times the head gasket is OK) for reassembly. Hope this helped.
Mac
