I don't know a thing about ASP engines but I was curious and went on eBay where I found one. It's a 2 cycle. If it has a piston ring I would start with an inspection of that. The ring could be broken into short pieces. The wrist pin could have loosened and scored the cylinder wall = new piston and cylinder liner. The connecting rod could have broken. I doubt that the loosening of the muffler would have caused you to pancake your aircraft but at least you saved the muffler. Two cycles aren't too challanging to disassemble. Pull the head and inspect the cyllinder bore. If you can turn the prop and see no movement of the piston with the head off it's got to be the connecting rod or a sheared-off connecting rod journal on the crankshaft. You can also view piston movement through the exhaust port - it's the quick and dirty way.

As far as the muffler screws go try using medium strength Locktite on the threads. If you use high strength Locktite you will need dynamite or lots of heat to remove the screws.

For some reason my fellow RCU members seem reluctant to reference eBay so I won't give you the item # but I would advise you to check it out. My apologies to the RCU staff if I have overstepped myself.

Good luck.

Bill