You don't need dummy batteries, if they're opposite facing like that they're series so all you need is the first + lead and the last - lead and you solder the power cord leads on, just make sure you get the polarity right. I just wired a set of LED desk lights that ran on 3 AAA batteries using only two wires, and no dummies. The current requirements for a motor are going to be high, test the power supply before you solder it on. 2 amps like KMoffett suggests sounds good just don't do too much more as the motor will use it at the start and it could be bad for the unit.