I've made decals using laser printer decal paper that's available from most office supply stores (i.e. Staples) or online.
If you take it to a place that has a color laser printer, they can do it in color for about a buck a page. The ones I made were for oil filter tops (there weren't S-2 filter canister decals available from the usual vendors.)
Wouldn't be hard to make a valve cover decal. You could make it say whatever you wanted.
