Mne was Chevrolet red-orange for about 12 years (maybe 13 or 14). Repainted the center sections black a couple weeks ago, painted the side cases aluminum (with pearl high heat paint). Scraping the head/head cover fins clean on their edges.
I use 500*F paints. They don't smoke as badly when curing as the 350*F paints tend to. The fumes off heat cured enamels are pretty strong.
For the exhaust system, use BBQ grill or woodstove paint. Expect to recoat it a couple times, the finish tends to solvent pop when it is curing if the base wasn't perfectly clean and lightly sanded before painting.
When I start reassembling my antique Honda race bike turned street bike I'll likely be color matching the outer cases to the fork sliders and the tins. British Racing Green. The rest of the engine will be black or silver, likely black inner cases, head and head cover, with a silver cylinder bank ( old parallel twin sloper 250).
Look at older custom motorcycle magazines from the mid 70s to mid 80s for your engine color scheme. Or go here...
http://www.dotheton.com/forum/index.php?topic=28778.0 currently 10 pages long, some of the photos are long gone but there are a few absolute stunners...
The pic below has about the right amount of paint to alloy showing. I have one very similar going in one of my rides... Not a bike engine, but a good reference anyway.