You can go to the stealthbikes site in australia and find out (or contact them) what was on that serial number bike when it was made..
The size controller, battery, and motor you need are determined by the job you need the bike to do for you, under the conditions you need it to do that job, and for how long / far it has to do that job.
If you already have a battery you're going to use, then your controller size is limited by the battery's capabilities, because it has to supply all the power for the system. If you use a bigger controller than it can handle, it'll either just shutdown whenever you apply that much throttle if the battery's BMS is working and able to protect it, or could be damaged if it can't protect it.