I tried using DupliColor metalcast paint which depends on the underlying metal to provide the metal look. The problem is with pitted metal you see the pits. The upper square and head are the original color and the lower is the metalcast. I expect the shroud to be worse and if it is we may have to go with an opaque color. Or maybe do the block and head in metalcast and the tin in opaque black. Metalcast paint will take 500F intermittently. A web video shows head temperature is about 250F. It also has the advantage of no primer.
But when you have pitted metal using it as the visible base is not such a good thing. They do offer a base coat that I want to look into.