Welcome to the club Jon, sadly the web site is slow to take off but there are a few of us participating. 3rd gear problems seemed to be more common with the ETZ models which use the same gear assembly and it was popularly attributed to wear in the MZ machinery which produced the selector drum, there are many articles on the subject in the Hints & tips booklets which are available from Regalia. Supa5s did not often suffer from this problem in there early days and I wonder if your engine was made towards the end of the Supa5’s life (around 81/82 though many were not registered in the Uk until much later).
I spent some time recently wrestling with a rogue gearbox and leant a few new things along the way so I kept a diary of my experiences. let me know when you get to the stage of tackling your engine/gearbox and I will tell you how to download a copy if you are interested.
The engineering drawings for the Supa5 engine are in the workshop manual but really all you need is a good quality clutch puller and the bridge used to separate the crank cases. Go for the puller advertised as workshop quality, the smaller cheaper ones will eventually let you down and never ever try pulling the clutch with a 3 legged puller.If you really want to make your own, I can scan and email the drawings.