I painted my Moto Guzzi engine with POR15 engine enamel. It is available from Frost Restoration.
I degreased the parts with Jizer, took the corrosion off using Machine Marts own alloy cleaner, gave all the casings a full hot cycle in the dishwasher then treated them with POR15 Metal Ready.
The paint is quite thick and didn't brush on well but I thinned it with white spirits and it sprayed on very nicely. In the 6 years I owned the Guzzi it stayed on perfectly and resisted petrol, degreasing solvents and, on one traumatic occasion, battery acid.
As soon as Photobucket sorts itself out I will post before and after pictures.