I've got some huge drum brake hubs, but they aint mag. They are spoke. And very heavy. Off the top of my head I can't think of any mag wheel drum brake motorcycles. Scooters, plenty... thats where you should look. IIRC with MC's the switchover to disc started in the early to mid seventies, while the mag thing caught on five years after that. And unlike MC mags, scooter and moped mags might actually be light enough to use.
The cush drives in disc brake rear mags can be pretty large though, so it may not matter. Depending on how much room you need you could machine out the cush drive section. Another interesting thing to have a look at is shaft drive motorcycles. Those wheels may work, but all of these options are way heavy. I have a couple of shaft drive rear wheels I can take a look at tonight when I get home. I think one is and 18 and one a 16.
Unless you've got a free supply of motorcycle hubs and access to machining equipment I don't see this as a viable option for an any electric. In any case isn't this problem pretty much solved already with the use of thicker spokes on ebikes? For a full size e-motorcycle it is interesting to think about though...