Well, how about somebody find out for sure. Find a long 7% grade, take your Zuma 125, whack the throttle wide open and go for it. See how long it takes the engine to blow up. I didn't get very far, my engine was pinging, rattling, and knocking (same as trying to ride a motorcycle up the hill a couple of gears too high) I turned around quickly to save the engine. I even went so far as to install a clip on tachometer on it, and found the rpms while trying to climb this steep grade were less than half what they were riding on a level road at 55 mph. RPMs should have been nearly the same, as climbing the mountain put a lot more of a load on it than riding full speed on a level road. If gearing is not the problem, then how do you explain it?
2002 Vulcan 750 (being repaired, engine has to come out) 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch moped