Originally Posted by mapbike
I've made half moon/woodruff keys from Grade 8 flat washers before and from grade 5 flat washers which is probably best since you don't want the key to be so hard that it destroys the shaft its in rather than just shearing.
That's a good point...
I was going to suggest borrowing the key from the bevel gear and putting it in the missing spot on the clutch shaft, the bevel gear won't slip without a key due to it's taper fir, but it needs to be torqued down to about 10 lb/ft using a bolt and washer in place of that screw.
When I put the centrifugal clutch on mine (until I broke it) there's no key for the shaft extension so just the torque on the bolt and the taper fit holds it and keeps it from slipping. I doubt you could get the screw that's in it now tight enough so if you do try this, find a bolt with the right thread size and a washer, it does need to be fairly tight.