"...Might go ahead and order a MTD Kit..."
https://www.amazon.com/MTD-791-18001...030277&sr=8-19
That same part number is used for this Ryobi nut/washer kit.
http://vi.raptor.ebaydesc.com/ws/eBa...=1611095861157
But I don't have a clue what size shaft it fits. The same kit also fits some Cub Cadet, Troy built, & other brands/models.
https://www.cubcadet.com/en_US/blade...1-180014B.html
https://www.troybilt.com/en_US/blade...le-specs-chore
Couldn't find any specs on either the nut or washer in the kit.