I had a '79 Magnum MKII years ago and had to replace the "short clutch cable". Its a lower cable linkage inside of the case. Not fun to get to, but it can be done. Double check everything else before digging in to that.
At the time I couldn't find a replacement so I cobbled one together. Looks like you can get them now... https://www.treatland.tv/puch-ZA50-starter-clutch-adjuster-p/puch-za50-starter-adjuster.htm