Good news and bad news. You can buy a 1/4 x 20 thumb screw at the hardware store.
Bad news, you can;t sue the 90993 ash sweeper with that. You need to find a 7409.
They are still out there, just hit some Ace hardware stores.
The handles will clean up with some light sanding and then your favorite stain\sealer.
Not sure whats holding it together, my guess is that the blades broke off and the large bolt that hods them to the bow land the cross bar is all that remains. That's part of the 7409 kit.
Enjoy the project!
AJ
Pic of a 7409 Kit You don;t use the washer with the MT, the crossbar acts as the washer.