I got this photo of a Corvair Converted Zundapp KS601 from the ADV site. I think it still utilized the original Zundapp transmission so that somewhat attests to the strength of the transmissions.
The Zundapp Transmission chain configuration. I have a friend who had ridden one years ago and he said it felt like a rubber band. The manual shifter engages a system which automatically shifts the gears, The lever does not directly link to the gear set.
If for some chance my Zundapp engine was completely shot I might entertain the thought of a BMW engine coversion. I have the 1982 R65 which could be a potential donor but.......I'd hate to destroy one of the few remaining KS601's. If I did do a conversion it would have to be reversable to the original drive train.