From everything I've seen and heard, the ZA50 will do just fine with a moderate kit on it. I rode yesterday with a guy on an '85 Puch 2-Speed that was had:
It RIPPED. The trick? He said you just drive it normally through 1st gear. You can crank on it, but when it goes to shift, you back off a bit on the gas....don't let it jamb at high revs. He's had it for over 200 miles now and it runs great....up to 50mph...or more.
You could conceivable throw some heavier clutch springs on it.....but I don't think you'll even have to do that. It's a PUCH. They're built to last.
Obviously an E50 would leave you with zero hesitations....but you shouldn't have any issues kitting a ZA50.