YouTuber Motocheez has done a great deal of hands-on research on cheap Chinese motorcycles. One of his latest projects, a Honda Grom clone known as the TaoTao Hellcat, has worked out pretty well, but Motocheez wanted more. So he followed a recommendation from the internet, because that always works out well, and pulled the old muscle car trick of dropping a (relatively) big engine into a small frame.
For $700, Motocheez picked up a Zongshen 190cc engine. It's quite similar in design to the Hellcat's original Honda 125 clone. It's carbureted like the Hellcat, though it can be drilled out for a fuel injector if you're installing it on a more sophisticated bike. It also has a five-speed transmission, a nice upgrade from four gears on the stock 125. Physically it bolts right on, but as usual with these more affordable components from China, there are a few quirks to work around.
A well-documented "feature" of the Zongshen 190 upgrade is an issue with the shift drum star gear. Push the shift lever down too far and it will pop out of position, resulting in the bike being unable to stay in any gear. Two strategically placed washers can fix this problem, but Motocheez opted to leave it alone for now and see what happens. The starter chain does not come pre-lubricated, but this is easy to take care of as long as you know you need to do it. The stock head pipe needs a little bending to fit the bigger engine. Motocheez made it work, but if you're going through the trouble of a big block conversion, chances are you're not keeping the original exhaust anyway. There is no wiring harness, so you'll have to figure out which wire goes where. Fortunately, there aren't many wires to figure out, and a strong online community to help. Finally, although the engine comes with a kickstarter the original 125 doesn't have, the lever doesn't fit on the Hellcat. Fortunately, the electric starter works just fine. The stock clutch lever can work, but you're better off replacing it with a slightly longer one.
All that said, the 190 conversion works, and works well. Predictably, it has a great deal more torque. You don't need to wring out the engine to go up every hill. While the stock 125 tops out around 50-55mph, the 190 will easily maintain and exceed 60. That's with the 15-tooth sprocket it comes with. The engine certainly has enough oomph to run the 16-tooth sprocket of the original 125 for even higher speeds. Surprisingly, the PZ28 carburetor that comes with the Zongshen engine is jetted perfectly for otherwise stock running gear, unlike most engines that are jetted too lean.
A $700 engine with no warranty isn't exactly chump change, especially considering that the entire bike didn't cost much more than $1,000. Compared to a $3,399 Grom, or a $3,199 Kawasaki Z125 Pro, you're still talking half the price for a significantly quicker motorcycle. For someone who's handy with a wrench, it's mighty tempting despite its issues.