I had a buddy that had a Sachs/DKW 125 back in the early '70s. The engine was bulletproof, . . . that thing got thrashed all summer long in the SoCal desert and never whimpered. So I'd guess that the original design on the "Flying Dutchman" is sound, but be careful buying something if you don't know where you will be able to find parts. Unless you are a tinkerer, and maybe have a friend who's a machinist, you could end up wth a $150 boat anchor. Just my 2 cents.
-kevin