If Andy is right, it would be well worth seeking out a Sachs engine-gearbox to suit. Not often you come across such a complete and interesting little rolling chassis that takes a motor that is (potentially) procurable.
It just so happens that I have a Maico in the shed: a 1956 ISDT Replica. This was a factory-built model, based on the 250cc Blizzard but with many alterations for off-road use. The cylinder, for example, has the pipe coming out the other side, while the leading link front fork, wheels, etc. have various mods from the road version. In the mid 1950s the Maico was one of the hottest off-road bikes available, and their models remained competitive well into the 1980s. Highly sought after now.
I think a pre-war Maico lightweight, being the predecessor to the fine later Maicos, would be an interesting and fun thing to have, and I'd gladly have one in my shed.
Cheers
Leon