MkII Service model screw

[WebleyWombler]

I recently bought a Webley MkII that has been heavily refinished but have a problem with it. The front screw that holds the trigger unit to the block has stripped threads and the whole unit moves away from the gun so it cocks on the bear trap only.
The screw is a cheese head and Is O.159" diameter which puts it in the 3ba range but that size won't hold so it looks like I will have to tap the boss out ( not a lot of meat there) and make a new screw. Looks like no.8 unf or m4.5 is the way to go and I have the metric tackle but not the UNF stuff.

I would like to tap it an SWG thread but the tackle is hard to get and I only have a few of the smaller sizes. No8 would be the ideal size to go to if I could get the gear.

Is it wrong to put ISO metric threads on a near 100 year old gun, and does anyone know the screws original diameter and pitch?
Making screws is easy, it's choosing the size that's difficult

[Troubledshooter]

Here's a scan of the relevant drawing part.

[WebleyWombler]

Excellent David. Thank you. I'll buy a set of No.8x36 unf and try cleaning up the female . A new screw should hopefully sort it and I won't have to tap a size up.