The part in the picture is the magneto rotor; condolences on the occasion of your major rust issue.
You're going to have to let some penetrant work on that center nut for a while, maybe a week. Spray it down every day with something good, like Kroil, and let it soak.
Once you can get the nut off, you need a special puller tool to extract the rotor. Once it's off, you can survey the wreckage behind it.
In the meantime, read one of the Puch manuals on mopedriders.org and find out if that nut is a RH or LH thread. That will save you some frustration. Good luck with it, my Columbia is in a similar state.