What is this?

on engine: sachs mot.nr. 8577125 hub-r 245 cm2

I've looked up Sachs bikes to find something similar, but not having much luck.

 

[Deleted member 12676]

that is an early model KTM (250 or more)
Like Penton they started with Sachs engines.
I had a Penton 125 and it had one. good engine but the shifting was terrible. lots of false neutrals.
I hope you fix it up!

 

[wheelbender6]

If its a KTM, I guess the owner didn't like orange too much. Nice dirt bike.

 

[Deleted member 12676]

Way back then the KTMs were all different colors (red, white, blue, etc)
see http://www.vintagedirtbikeparts.net/VintageKTMPhotos.htm