Evening Frode
The pitmans book of the Douglas 5th edition states that Lodge H1 are the requirements for both the OHV and side valve engines, the 6th edition states Lodge C3 for side valve engines. I have used both these in my 1936/37 600 SV units and find starting on a hot engine is easier with the H1.
New old stock items are hard to find, so from
www.gsparkplug.com a modern replacement for the Lodge H1would be
Champion K8G or NGK A4. And for the Lodge C3 from info@lsengineers.co.Uk would be Champion D14 or NGK AB6.
Good luck Steve