Helmut,
Would a Sach's motor in a Hercules be close.
Yes.It is a SACHS MC engine in a frame build bei Hercules for SACHS and therefore never called Hercules but SACHS MC Grand Prix.
This bike was created for the MX grand prix only.
A light weight frame (very much different then the Hercules GS frame),and the (at that time) pretty modern SACHS engine which housed a 7 speed tranny in all GS models,but came for the 250 MX as a 5 speeder.
Just by pulling a pin out of the shifter drum would make it a 6 speeder and just by adding 1 gear you had a 7 Speeder.This bike ,because of his better handling( and more suspension) and lighter weight was Quickly modified into a GS ( OFF ROAD) for the ISDTs.The horizontly split cases made a tranny or crank replacement very quick and easy.
Here the original SACHS MC 250 (ridden by Willi Bauer)
Here the US model raced by Frank Stacy.
Here a modif.Sachs for ISDT events.
My personal modif.Sachs for Canada in the 70s
my latest mods.Reeds and Nicasil and spec.pipe.