W did not get Harmony down here but in any case it is just a name and not used when searching for parts lists.
go here https://peparts.honda.com/powerequipment#/browse and work your way through the lists.
You will need the NUMBERS off the id tag and in particular the letters preceeding the numbers.
I did a parts lookup with model and serial numbers, and there are no diagrams available for the inside of the transmission. The parts diagram does note that it's a Tuff Torq transmission and not made by Honda. It's possible you may be able to find a Tuff Torq model number on the transmission case and use that to look it up...