Any BBQ or slow combustion heater shop has flat black heatproof paint which is what I used.
In a previous thread wise advice was given about being verrrrrrry careful and put a spanner onto the hex of the delicate oil cooler before undoing the pipe joints.
I took off the filter, gave it a good scrub in degreaser, suds, solvents then turps. Poured the paint on, brushed it into the nooks and crannies, it dries real fast.
No noticable effect on cooling, and any stone chips can be touched up easy.