I've used Eastwood's Stainless Steel High Temp Coating (10178Z). Works well but with any coating, you will have to remove the manifolds (easy to do on a M103) and sandblast them. This stuff brushes on and gives a final color and gloss of cast iron. The last can I bought was $24 for 16 fl oz.
I'd like to try ceramic coating a set of manifolds some day.