the thread is 16mm x 1.5mm which is a fine ISO metric. Depends where you live, but this filter thread is not super common, pretty much suit only Norton Commando these days, was used on some French Simca cars, and he, they are on every street corner, right?
I decided to use a GM size filter, as used on Gen 111 V8 engines for yonks. The filter and the oring dimensions are close enough to fit nicely, I used a 16 x 1.5mm tap to thread a blank piece of alloy bar, then thread cut the male thread, 3/4 UNF from memory in a lathe. Now I can go to the local auto parts place and buy the filter for $11
if you have a lathe or a friend with one, this is an easy mod.
Cheers Rich