Does it come apart or do I just buy a new one. I’m replacing the windshield so now a good time to fix this too

Bill -

You can probably get a local glass & mirror company to replace the glass, but not sure of quality workmanship that you'll get.

Porsche etc. still sell the mirrors new (same as used on 911/912/930), so why not go for a fresh unit, & save the old original as a just-in-case. I'm not sure if you're replacing with a generic XYZ Chinese windshields, or one of the originals from Porsche/P-suppliers &ProSource - but for the cost of that - the cost of a proper new mirror is justified, if your budget can do it.

Make sure to order 2-3 of the 3M double sided mounting pads as back-ups, since they can fail, or if the windshield fails.

Cheers!
Tom
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A friend of mine did it to save an early interior mirror. He dropped the mirror in boiling water, pulled it out and the housing was soft enough to pry back and remove the mirror. Got a mirror cut from the glass shop and installation was the reverse of removal. He wore leather gloves while handling the hot plastic and used a plastic body tool to help get under the glass.

Hmm I tried it once, w/o the heating application, wasn't too successful.
Billy given your stature. you might consider an aftermarket (articulating) mirror to get a better view out the rear.

I mount my mirror closer to the passenger. There are no visor to work around. They just block my view even when they are are up. When I had the section redone that goes across the to of the window, I had the visor are where the visors screw in covered over.

We did one for the 914-6 restoration we just finished.
Had a piece cut. Either was too thick or too thin. Went with thin. Snapped during assembly.
Sooooooooooooooo we sent the old glass out for remirroring. Gooder as new.

Cary nailed it. US glass not acceptable. 50+ year old plastic may not be to malleable to even heat.

Great idea Mark !
I recently attempted to remove the glass with brute effort and stopped before
injuring myself.
I've often wondered how they were assembled.
I'll try heat next time.
Future fingers crossed.
I'll also use the opportunity to tighten the inaccesable screw within.