I am part out a Honda GX670 engine that was a repower for a John Deere 420 lawn tractor.  It is up to you, and you alone, to determine if this part fits your engine.

Honda GX670 drive shaft coupler.  This is in excellent shape and worked perfectly as shown and from a Deere 420.  It mounts to the flywheel and connects to your Deere drive shaft.  It is 3 11/16" in diameter and 3 1/8" deep.  Thanks.