How I fixed the problem:

Pictured is the chainsaw body (upper right) with the 2 8mm studs protruding. At the bottom are an 8mm regular nut, an 8mm connecting nut (about 1" long), and the old assembly knob. In the center is the clutch cover with the existing recessed hole on the left where the assembly knob used to fit, but now takes the 8mm regular nut.
I didn't want to make a large recess (on the right) for a socket to fit into, so I shopped around for an 8mm coupling nut that would stand off sufficient for me to fit a box wrench over it. I bored out the recess just enough to fit the coupling nut.
Here is the final product:

It's possible that I could have solved the problem with just tightening the single nut, but the 2nd nut gives a little extra holding grip.