Please can I find out if looking for the specifications for a controller does the max rated amps allowed change with motor watts?
The motor watts is a meaningless number. It has no bearing on how much power the motor will give, nor how much it can handle.
The controller maximum amps determines the maximum power from any motor. The battery volts also affects it in direct proportion.
The power you get is approximately in direct proportion to volts x amps. If you want more power, it's always better to use a 48v system. remember, if your system allows high power, you can always turn it down, but if it doesn't allow enough, you can't turn it up.
It might be better if you say what you want the bike to do, then we can recommend a motor and controller.