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?
I won't spend more on a bike than I think it's worth, but if it's a good deal, I don't seem to have a problem buying bikes I don't need.
2002 Vulcan 750, 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
1980 Puch moped