@saadleo: 1989 Yamaha (non CDi hope i am correct) Yamaha 100 from 1985 - 1992 is a killer if in original condition.
I will suggest no need to do any modifiction it will not help, cause the the carborator of these bikes in inside the engine, and if you but a bigger piston than how will you make a place for the bigger carborator ??? there no way you can install the any other carbi than the similer one. Also the suction plate will get messed up and you will end in a worse condition than the current.
RX's CG125 and all which have carborator outside like CD70 are easy to get modified.
Hope you understand.
I learnt flying motorbikes on Yamaha 100 1986