For your head bolts - look in the parts list. I'm fairly sure the dimensions are in there, or at least the thread size. Then talk to Clay Jones at http://www.acmestainless.co.uk/Ariel.asp
. He should be able to help (might know the right size anyway). Alternatively give Pat Rowbotham a ring and ask if he knows (Black Ariel spares).
Ariels - 1913 TT Model, 20 Roadster, 28 Model D, 30 Model B, 38 VH, 52ish VH