I’m in the same situation with my 1927 Flyer which has a similar exhaust system. I was planning to use ‘SMC / Techline black satin’ which I found via Google. Claimed to be resistant to 2000c it appears to be a paint on ceramic coating, not cheap at £26.89 for 4 oz. Available from ‘https://www.rust.co.uk/hightempcoatings.cfm’. I haven’t purchased it yet but will provide feed back when I’ve done so and come to some sort of conclusion (unless anyone beats me to it!).