Wow!
Sounds like at least a couple different problems.
Super loose chain part means like hanging down??? could be rag joint, wheel bearing, loose wheel, bad drive sprocket, or internal support for the drive sprocket in the case, loose engine mount, maybe even a broken frame.
Really jerky for a second, then dying sounds like you have seized the motor.
Start with the motor mount inspection, then wheel & rag, the pop the clutch release cover off and see how the motor turns by putting a socket on the nut holding the gear.
U could also drop the exhaust pipe and look in the port at the piston to see if there are obvious problems in that small window.
Always gotta look for the silver lining...
Great little project to learn about this kinda stuff !
Good luck
rc