help me sort this out

looks like there are 2 choices for getting more power/pull from an engine. Either swap out the motor or change the battery 'hook up'

First Question (BTW, thanks for the info guys and if there is already a thread that I couldnt find, I wont be offended if you just refer me to the thread, no need to reinvent the wheel)... I dont know what battery combo provides the most power. I have one of the Shinkansen trains with the 2 speeds and it uses a C battery, and I have a Diesel that also uses a C, but it sounds like these can be modified with more battery power? Whats the breakdown is 2X AA better than C and all are better than one AA? Side note, seems like every train has a different power set up, some are C, some are one A, some have the battery in the 2nd car, some dont.

Second question is about swapping out the engine/motor. I'm all for a little easier even if the motor costs alittle more. It sounds like I can just buy a motor and swap them out (w the help of a soldering iron). Is that accurate? any brands/ types recommended? i saw this lot on ebay, would these work?

Thanks,
Kevin