UNF is an option John, although the g/box/primary chain adjuster set screw mates with a BSC tapped crosshead which spans the left and right hand engine and g/box mounting plates.
In the eBay link you gave I didn't find UNF set screws (g/box adjuster) and they don't list a 5/16" dia bolt (g/box top bolt) for the other one.
I realise that the eBay link you gave is just an example so a further search for UNF equivalents may be more fruitful than persisting with BSC.
(I appreciate all the suggestions although no doubt like others I'm finding this tedious searching about as exciting as having root canal treatment).