My TS250/1 Supa 5 has ETZ 250 forks and disk brake so I can’t speak from actual experience but looking at the TS250/1 parts list on this site, on the drum brake wheel, there does not seem to be anything called a bearing seal.
The exploded diagram shows 2 bearings, a distance tube and 2 packing rings (part no 01-24.314) – are those are what you mean by wheel bearing seal?
Presumably you can use sealed bearings as well, but I note that ‘The MZ Shop’ lists ‘packing rings’ under the 01-24.314 part number and has them in stock.
Other MZ parts suppliers may also have them – try Martin Burwin – contact number is on his advert in the Club magazine.
Regards
Andrew