I guess you're talking coaster brake here- good luck
I searched ebay and found some new asian stuff listed in freewheel hubs- nothing fancy- steel hubs, but not a lot of money, and grooved so that a sprocket would fit over them- they'd still have to be secured with a rag joint-
but then you could fit handbrakes
Metal 36 Spoke Holes Back Rear Hub for Bicycle Bike | eBay
this second one is the kind that came on my Micargi multi-speed cruiser- the ridge stops the motor sprock a perfect distance from the spokes- the diameters on both these hubs is the usual motor sprock diameter-
I still look for a large flange flip/flop with smooth alloy sides undrilled so that I can drill them for sprocks with the 6 inner disk brake style holes and bolt that directly to the hub- hard to find but out there.