Hi Rob,
A decent engine reconditioners shop will have books containing hundreds of valve sizes
There will be some in there that will be a good basis for modification
I have modified Perkins diesel valves to fit several BSA and AJS models, 3/8 stem and 45mm dia head
Also ""Bantam John"" has blank valves in various sizes, around £20 a pop, these have long stems which are handy for SV engines
I have just had a pair of these slotted (spark eroded for £20) These are going into a side valve Blackburne!!
An alternative is to turn collet grooves and fit caps and collets from a more modern BSA/Triumph etc.
I picked up a couple of "gold star " valves at Stafford at the weekend, these are going to be modified to fit the BSA L25 ohv engine, these engines have really big valves for a 350
I will be using BSA A65 caps and collets
My 350ohv Blackburne engine has BSA A65 valves fitted
I made new guides to suit the smaller stems
HTH
John