The Yard-Arm uses a Type 184 (radial mount) engine, the clutch drum in that engine should have a keyway in it for the drive shaft, the Yard-Arm parts list doesn't give the part number for the rear housing but it looks different as it has to fit the drive tube, that said unless one appears we don't know if there could be an older version that uses a different engine Type.
The Model L was one of the standard engine range, available for anyone to buy & fit to whatever tool or application they could think of, the Type 99 & Type 199 are the later direct drive engines with clutch.
I have since acquired a Type 106 engine with the clutch assembly fitted, no idea if it's original yet (engine is sn 052178).
David