Brushed VS. Brushless, brushless is more efficient.
Most hobby grade ESC's will run multi-pole motors little car motors are 2 or 4 pole & big airplane motors can be 12+ pole. "Pole" refers to the number or groups of electromagnets in the stator.

The thing that comes to mind is really how to make the conversion to have a simple switch to control an ESC that is looking for more than just on/off...

If you have a bunch of spare parts lying around & you are trying to make it work I'm sure something can be done. You can run old controllers off of modern LiPo's but you'll want some type of LVC or alarm so you don't go too far...

You may have better luck with some type of consumer grade tools & batteries like some Ryobi Lithium 18v drills with batteries & butcher parts off of them & series them to 36v to use the batteries? I think Dewalt has a 56V drill too.

have any pictures or what you have & are trying to accomplish?