The new sprocket called for a rear disc brake. The braking surface on modern band brake drums is some 1/4" wider than that on the stock welded on drum. So I ran into clearance issues with the modern stuff and in the end chain alignment issues. The band brake had to go.
With the trunk in place there is just enough clearance for the rotor. There are a couple of calipers available: a conventional single piston in the photo below and a much heftier one with a built in parking brake. The Microcar is missing the parking brake. It remains to fabricate the caliper mounting bracket. I'll bet the parking brake on the bigger caliper is operated by some sort of a lever to lock it in place.
Rear brakes are all foot activated on go karts and obviously motorcycles because a lot more force is required. So my rear brake setup calls for a master cylinder, pedal with mounting rail, mechanical linkage to the master cylinder. All that stuff can wait because there's a disc brake in the front.