i dont know alot about zipp's apart from the 50cc one's so i couldnt comment on that but the runner with 172 kit on is amazing fun but i dont think there very realiable
172 Runners can be perfectly reliable if done right. I've had loads of 172s and ridden them quite hard for many thousands of miles. The actual engines have never failed me once. Only issues I have ever had is electrical faults, or just generally making sure they're set up right. If you tune it to the absolute maximum possible then you will lose reliability, but you can have a lovely quick 172 that is just as reliable as a 125. For good fun 125s either a Runner, Typhoon or Aprilia SR are great. I'm partial to Italjet Dragsters myself, but don't think I could recommend one really. Pain in the arse getting parts for them.
125 4stroke zip 100cc 4 stroke zip 50cc 4 stroke zip 50cc two stroke zip. Only zips out there. Unless you buy a bodge.
125 zips how fast are they and what can be done to them? And the 172 runner sounds good but it's finding one