Ian, your Puch uses the same crank arm / pedal system that was used on low-end bicycles until the early seventies. So-called Cottered Cranks (which I now need to register as a band name) aren't bad, necessarily, they're just heavy, and have one big drawback:
Most people, it seems, tend to service these types of cranks with a hammer. I mean, look at your remaining crank arm - it's just begging you to knock out that cotter pin, no? Once you do, though, you can't put it back in; that's been the end of a lot of otherwise perfectly good bicycles (and, often, the beginning of new ownership).
So, now that you know the secret to bicycle/moped parts compatibility, you can make a few educated guesses. Does it look like the previous owner tried to replace the crank arm with one from a bicycle? This arm would very likely be much straighter than the one you're looking for.
If that's the case, then perhaps the solution to your problem is as simple as finding another moped-specific bent crank arm. If it's not, then you'll have to wait for another poster who is both familiar with the za50 engine, and not jealous of your awesome vintage ride.