My last spring starter is currently buried in an airplane,but if memory serves the back end of the crankshaft is threaded and the spring sleeve is held in place by a second threaded sleeve or bolt.It locks on the end of the threaded portion of the crank leaving the spring starter sleeve free to rotate.
The stripped driver must be fixed by one of the standard machine shop methods,go to the next bigger bolt and rethread,buy a new driver,fill the hole with weld and redrill and tap etc etc.....but the easiest fix might be to replace the six stud setup with a single bolt or nut .....I bet someone like RCIGN could/would fix both problems for you...or suggest someone who would.
Is the photo of the bipe the one with the Sachs in it?Looks v nice.Plans?Kit?