Your bike should be of standard quality or better - cheap, lightly-built push bikes are not suitable.  This includes light aluminium forks.

Ensure that the front forks are at least 90mm wide

How to determine the size of the wheel you need for your bike:
The wheel size is the rim with the tube PLUS tire installed and inflated. You can also look for the wheel size on your bicycle tire.

The kits are designed as a device for alternative/supplementary power - they are not designed to convert your push bike to a high-speed, endurance motorbike racer.