The stepper, by my experience, will maybe make 20 to 50ma at 12V.
The scooter motor may make a little bit of power, like a couple amps, but the spped is usually so high on them that they are not practicle to attempt (except for fun).
50W is pretty much up into Ametek class.
Might try a UK-type treadmill motor, if you FIRST read honest reports about what actually works.
Everybody selling a treadmill motor thinks the one they have is perfect, but very few are suitable at all.
For 50W out of a motor, think about things that weigh 3 or 4 or 5kg.
G-