Since many engine modifications are driven by racing interests, the hardware that's available tends to reflect what's allowable in the racing world. In the case of mini quads, racing classes are basically divided into three groups: a 0-50cc class, 51-70cc class, and 70-90cc class. The performance aftermarket has set their sights on the 70s because the other classes are already maxed out sizewise in stock configuration. 

The aluminum cylinder (right) weighs about a third the stocker (nearly a 2 lb savings).

As a gross approximation, your incremental power improvement should be directly proportional to the displacement increase. In other words, going from a 50 to a 70 is 40% bigger. Then, assuming that porting hasn't changed and that the intake/exhaust can still handle the extra air demanded by the bigger piston, you should end up with about 40% more power. 

In the case of the aluminum 70cc WRH cylinder, kindly ignore the simplistic theory above. How's nearly 100% improvement sound? Talk about a Halloween treat, our project Alpha machine has been transformed into a truly scary-fast little quad! By installing this cylinder the power of the machine was literally doubled, which is a pretty impressive feat considering that it was already running pretty well. 

This trick nikasil-coated cast aluminum cylinder is not just a lightweight facsimile of its heavy-set cast iron cousin. It's a radically different cylinder and piston with altogether different porting. Drawing on the developments of the European scooter racing industry (yes, knee-dragging scooter road-racing!), this kit has a fair amount of R&D behind it. After all, the Asian mini-quad has its powertrain roots as a scooter which has been around for quite some time. 

Incredible power increase, as measured at the rear wheels on a chassis dyno. (click to enlarge image).

Although you can perform this modification without the stage 2 exhaust pipe installation, likely you won't see as much of an improvement as we did in this case. However, if you're interested in recreational riding and all you desire is a 40% bump to your 50cc machine and don't want the accompanying racket of a hi-po exhaust, then this kit or the cast-iron version could very well be what the doctor ordered.

Tips- 

• thoroughly power wash the entire engine area so as to avoid dirt falling into the case during disassembly
• turn the quad up vertically on its rear wheels to make it easier to work on
• make sure to install the piston with the cutout to the rear (alum kit)
• don't forget to lightly oil the cylinder, ring groove, and piston pin before assembly