I think I read somewhere that the motor is a 370 size brushed motor.

Good luck getting an 8.4v battery to fit in the wand though. They're only made to accept a 6 cell NIMH, unless you have the Old style EZ start in which the button case just zip ties to the battery. Either way I'd just take that EZ start off and throw it in the garbage and switch to a pull start. Much easier than having to wait an hour for a NIMH to charge. All you need is a glow ignitor and you're good to go.