my feeling is that people are upset because magnums are a "good" moped. there are plenty of motorcycles and not so many mopeds. so, why tip the balence in the direction it is already tilted? nobody cares about batavus top tanks because you have to mount a new engine to make them fast anyway. with a puch you can use what it already has.
it's your moped do what you want with it. your biggest problems will be finding a rear wheel that fits and making spacing work, mounting a pipe, and the law.
these frames are strong enough for 5 more hp. the brakes are weak but can be upgraded with some modifications.
i hope you choose to do this on another moped. maybe someone with a batavus or sachs or something would trade you.