Okay, I will contribut what I know about this, which is incomplete. However, I will say that you can do a search on this because it has been discussed previously.
First, the engines on the Puchs were interchangeable and should bolt right onto the Maxi or Magnum frames.
HOWEVER...while you can use a Magnum top end on a Maxi (and viceversa), the exhaust pipes will NOT work as readily. If you use the Maxi top end, you'll need to use the Maxi exhaust (and viceversa, as I was informed) or you will have to make modifications with welding, drilling etc. Puch did make a one speed Magnum (I think they called it the "Cobra") and if you look at it, it has the Maxi style exhaust rather than the larger Magnum exhaust. If you do a search, I believe Mopedlar got one of these running within the last few months and posted a photo on this website somewhere. He can tell you way more about this than can I.
But at least it is a start.
And no extraneous tuna shots.