A higher compression ratio will make the engine more powerful, but it may also make it more difficult to start and more prone to knocking. Therefore, it is important to make sure that the engine is properly tuned when running an LT185 cylinder and piston in an LT125 engine.
In addition to the cylinder and piston, you will also need to use the LT185 head and gasket in order to complete the conversion. The LT185 head has a different combustion chamber shape than the LT125 head, and the LT185 gasket has a larger bore size to match the LT185 cylinder.
Once you have installed the LT185 cylinder, piston, head, and gasket, you will need to adjust the ignition timing and jetting in order to make sure that the engine is running properly.