Loose flex plates or broken flexplates happen. Especially if years back the builder drilled out a factory plate. We run SFI certified plates today on the commercial drives and I have seen some Stinger users with a Billet plate.
We use LocTite on the flex plate bolts and torque to spec. Think we use 50 lbs as the setting.
Thats my Drawing I scratched it out on the counter. The gear box makes a clickety clack noise I love. Need a vent tube and oil - any aircraft oil or equivalent. Nothing special about the box itself it's the mounting plates that are special. Really almost any Lycoming aircraft gear box would of bolted up to my old setup