With that 0.67 inside diameter, you'd need to find tubing with its pilot hole size around 0.67. For 1/2" NPT (taper thread) pipe, it looks up as 23/32 (0.718) and for 1/2" NPS (straight thread like a bolt) pipe, it's 47/64 (0.734). Where ya gonna get That Drill, That Tap, and when you do, you risk cracking that pot metal piece that already broke. You'd also have to enlarge the hole in that spring retainer (the gold piece), although a washer would probably do at that point.
Why not just get a pipe nipple or lamp tubing and EPOXY it inside the 0.67" diameter and get on with assembly?
I looked up "lamp pipe" and the 1/4 IP (Iron Pipe) is 1/2" outside diameter. To me, that's sloppy in the 0.67 diameter it epoxies into.
Remember, I'm suggesting PIPE only because some of us have mirrors with wiring going through the Arm and into the Head. If your mirror isn't power adjust, heated, or lighted, get a hunka 5/8" all thread plus a nut and rock on. I probably have a piece of that I'd mail to you.
If you're wondering, I have nothing better to do at the moment than develop this repair... Latest thought:
1. 3/8" Pipe Nipple (or 5/8" threaded rod) and Nut. Nipple short enough to fit down into the 0.67" hole and hold the compressed Spring without bottoming in that hole.
2. Leave Nut with a couple threads for adjustment.
3. Assemble and apply EPOXY (like JBWeld) to Pipe Nipple and 0.67 hole.
4. Compress with Vise or Clamp till Assembled Mirror can swing properly, holding on its detents under Spring Pressure.
5. Allow epoxy to cure. Remove Clamp.
6. Make final adjustment with Nut on remaining Few Treads.
7. Drip RED Loctite onto Nut Threads.
8. Trim protruding Nipple as needed.