A couple of things lately brought me to post this.
Six of the bicycle motors I have or have had, had a natural finish on them. One has paint....I didn't figure that out until I was cleaning it.
I clean all bicycle motors prior to giving them the once over for a couple of reasons...one is it's a lot nicer to work on an engine thats not covered in some funky Chinesemoline that smells almost as bad as the cheap grips that come with some of these kits. Another is if you don't clean them, that stuff bakes on to various degrees and looks like **** after a short time.
Polishing them is labor intensive and a specific look, enjoy!
Painting them is easy, and uses no special paint. Just be sure to clean the bicycle motor thoroughly with a strong solvent first. You don't need high temp. paint! I have painted with "regular" Krylon with exellent results. The only place the paint won't hold up is the first 6" of the exhaust, it will turn a funky grey. Most of the time you'll leave the pipe chrome anyhow. Clean the engine, and spray with your favorite aluminum paint for a refined look. Black for the vintage 70's look, or color match the engine to the bike if you went to art school. (camo doesn't count, see second picture
Finally, a nice charcol finish can be made using oven cleaner....follow the directions, or better yet, apply, wait a half hour, rinse THOROGHLY.
See the engine in the middle for an example of a "darkened" motor, still not cleaned up for mounting, but you get the idea.