It's setup for a keyway, but I believe I can just drill a hole to match up with the one on the shaft. I'm assuming that a 1/2 inch, grade 2 bolt will work in place of a shear bolt in this case. Does this sound reasonable?
1/2" (no matter the grade) bolt (unless you mean Wrench size) seems too big for a shear bolt on a 42" tiller. My 50" tiller has a 5/16" (maybe 8mm) shear bolt.