Assuming that you’ll be using the smaller ammeter used in most British headlamps for many years, it requires a hole 1.625” dia (~41.3mm).
The bezel surface that actually sits on the shell is about 2-3mm wide, so OD ~ 45-47mm.
If you can find or make any flat washer, ring or bit of pipe with approximately these dimensions (the ID of ~1.625” is the important measure) then you will be able to place it on the headlamp shell and see how good a fit you get in various parts of its curvature.
Just bear in mind that things inside the shell may interfere with the depth of the ammeter body.
If this happens to be the case, you could fill the ammeter with clear resin (with the needle showing a charge haha?) and then shave the body more shallow.