If you want a cpapcitor you need something over the voltage you are using, which is easy as it's only about 7.2 volts. Use a 50 volt capacitor. 0.1 micro farad capacitors are fine. You can use 0.01 micro farad, but it doesn't make a lot of difference.

Just get 3 of them for about $1, and solder them as you mentioned.

If you have a motor with adjustable timing, or a re-buildable motor, you don't want to solder to the can, as you will have to un-solder them every time you strip the motor. There SHOULD be a solder tab on one of the screws hat hold the end-bell on to the motor.