Instead of an actual jackshaft, someone suggested I mount a single, freewheeling sprocket on the swingarm pivot. That’s an excellent suggestion, and definitely worth considering. For right now, however, I’m going ahead with the jackshaft concept to see if I can actually make it work.
I picked up a dual-simplex #40 industrial sprocket in decent condition for $30. I enlarged the center bore to take a couple of 6203 sealed bearings. I still need to make the spacer that will go in-between the two bearings, plus a 17mm to 12mm sleeve to adapt the bearings’ ID to fit The Bride’s swingarm shaft diameter.