The really old cars used to use generators with brushes.
I looked it up on the internet and have forgotten the link, but the date that the cars started to use true alternators was about 1970. So the pre-1970 or so alternators used brushes and then pretty much everything afterwards is without them.
Since a brand new alternator can be purchased for $40 it seems like it would be better to go to the auto parts store and look through all your options and find the best match. However, if you already have an alternator laying around it would be worth checking it out.
I would prefer to buy the smallest and lowest amp alternator because then when you flip it around and use it as a motor it will drain your battery more slowly.
Remember that the MAIN reasons for switching would be:
No brushes means less to fail.
Induction as opposed to Permanent Magnet means that the powerband is wider and flatter. As long as you have the throttle wide open you will get good efficiency... the moment you let off the throttle is when the efficiency drops. It's better to run the motor full on or full off. (race mode)
...of those two the brushes part is what got me started on the idea, but it's the Inductance part that really serves the need for my Electric Bicycle Road Racer concept. The idea being that the "perfect" motor will run with maximum torque at a constant input power level. If the racing class is defined as "Formula 1K" then you are allowed a maximum of 1000 Watts of input power. A Permanent Magnet would not work as well under the fixed input power concept. (this all fits together in something much larger that I'm working on) If I ran the alternator at 48 volts and 40 amps that translates to 1920 watts of input power. (which is too much) So in order to satisfy my own "restrictor plate" racing I might go:
48 volts * 20 amps = 960 watts (effectively 1000 watts)
24 volts * 40 amps = 960 watts (effectively 1000 watts)
The sort of amazing thing is that I've had it set in the back of my mind that the Induction motor would be the "Holy Grail" of racing ebikes, but I somehow "never got the memo" that auto alternators were Three Phase Induction motors. That was a "wow" realization for me.