Try 21
and potentially even a little less timing. I think those motors had small ish rotors - certainly not the big stump pullers that the very first Orion's and Speed Passions came with. Would probably have run about 24 with those!!
Last winter I was running with the early prototype shell, that had the smaller air vents!
Sure it got warm, but nothing to worry about! That was with Nosram/LRP TC speedo.