Okay, first question: E50 (1 speed) or ZA50 (2 speed)?
Either way, scratch the 80cc Metra large port off your list. You can make it work, but I think it's a LOT more involved than you want to get. You would do better to do a smaller kit and do it right rather than just shooting for the absolute biggest displacement kit there is.
Beyond that, here are some suggestions for places to look. Read over and peruse this stuff and see if it helps.
"1977 Mopeds Puch E50 Engine Build Tool": https://www.1977mopeds.com/product/1078/Puch-E50-Engine-Build/ - maybe you don't want to pay someone to do it (or do you?) but this is a pretty neat tool for exploring some of your options. It won't tell you how any given setup will perform, but it's a start.
"ZA50 Engine Build Tool": https://www.1977mopeds.com/product/988/Puch-ZA50-Engine-Build/ - same thing, but for the 2-speed engine
"Puch tuning": http://www.mopedarmy.com/wiki/Puch_tuning - this article links to all the articles in the wiki about kits and exhausts and all that good stuff. Get click-happy.
"Performance tuning": http://www.mopedarmy.com/wiki/Category:Performance_Tuning - this is a great run-down of your general performance tuning options. There's a lot of other things you can do besides a kit (or things you can do to complement a kit) that will make you go faster.
"Spreadsheet": http://spreadsheets.google.com/pub?key=p_Ysahl1B79WHMUBbvCzNfg - this is a list of setups people have used. You can see what speeds people are getting with what kits, what carbs they're using, what gearing they're using... all kinds of good stuff. Just remember that everything is self-reported, so some of it might not be accurate or updated properly.
"Treats": http://www.treatland.tv/ - a great source for Puch performance parts
So, basically.... you could probably make a Maxi do 70 if you REALLY wanted to (and there are people who can go that fast on mopeds). Is that sane? Safe? I'll let you answer those questions for yourself. I don't have any experience with Maxis, but I know a lot of people put aftermarket forks and frame braces on them when they start adding performance parts -- the 30-year-old frames and forks really aren't meant to withstand 50+mph speeds. How fast is fast enough for you? 50? You can do 50 on 50ccs if you do it right. 50 is probably easier on a 64cc Polini (a very fast kit, BTW) or a 70cc kit, but I'm trying to make the point that displacement isn't everything. Also keep in mind that if you actually want to ride this on normal city streets, having good acceleration can be more important than having a high top speed. A ped that goes 70 is great, but if it takes a mile to get there, you're going to have a lot of pissed off people stuck behind you.