When I worked at Pearl Harbor Shipyard, we always used stainless fasteners when dealing with aluminum. Dissimilar metals are always a problem on our ships so a lot goes into trying to keep these metals separate when possible and to use "neutral" fasteners.
That being said, these bike motors are all made of cast aluminum. If you plan on doing regular maintenance and overhauls, I'd say change any fastener that comes in contact with the casings to stainless. This won't "stop"corrosion, but will slow it down immensely. Btw, even stainless fasteners will eventually rust when in contact with aluminum. Just not as catastrophic nor as quick as regular steel fasteners.
Nothing scientific. Just what I learned working with the stuffs.