But I didn't anticipate that one act of brave self-reliance (Happy Independence Day!) would devolve into a series of mini-mechanical tragedies. Nothing tragic with a Capital T, for sure. More like tragi-comic or trag-ironic. Or tragi-stupidic.
The initial clock battery surgery was a little daunting at first, but once you realize it's just a bunch of screws, plastic and plugs, it's certainly not rocket science. There's something unnerving about having disassembled the entire head unit-you can imagine what doctors feel like sometimes in mid-operation after they open someone up and all the inner guts are exposed and they feel like, "oh crap. Now I've got to figure out how to put this stuff back together."

But I managed to extract the brain and here's the offending part:

A trip to the watch/calculator/hearing-aid battery section of Wallgreen's and voila!

I completed the task at precisely 4:44 PM

Now comes the fun & tragic part: reassembly.
The first time I reassembled, I was left with a few, uh, "extra" screws." So I "redisassembled" (that should be a new word in the English language), found the un-screwed holes, and filled them. Then upon re-reassembly, when inserting the head unit screw just below the headlight, and despite YouTube warnings to BE EXTREMELY CAREFUL! and PLACE A TOWEL UNDER THE HEADLAMP TO CATCH THE SCREW SHOULD YOU DROP IT, and all the while thinking to myself, "Dude, just be careful. You're not gonna drop it. But it would really suck if you dropped it..."
I dropped it.
"Kerplink-clank-clunk," down into the black abyss of Vespa guts and bodywork she goes. Never to be seen again, despite an hour of magnet-fishing and body-shaking. A screw lost, and a screw loosed. Tragedy #1.
I resign myself to a ride to the local Vespa Dealer (Vespa of Ridgefield, in CT) to get a replacement fastener, which is not the end of the world.
But as part of my attempt to jiggle the errant screw loose from the bodywork, I fired my GT up, and noticed that one of the side plastic reflectors was jiggling madly. So I reached inside to try to squeeze-on the little push fastener, and snap!. Tragedy #2.
Well, in for a penny, in for a pound. I'd be making the trip to the scoot dealer anyways, so I'll just pick-up another unbroken reflector and 2 push tabs.
Thus, I buzz on up to Branchville, CT (about a 20 minute drive from me) to pick up the parts, when, to my dismay, Tragedy #3: Branchville Motors/Vespa Ridgefield is closed. Not for the day; forever. Dust in the windows. No sign of owners, no bikes, no anything. No sign saying "Going out of business." Or "A Note to Our Valued Customers." Just a banner over the Vespa-half's front door saying "All Accessories 5-50% Off." And, ironically, not a word about it on their website, or on any forums. http://www.branchville.com/new_vehicle_list.asp?Manufacturer=363&Category=12&CatDesc=Scooters
Well, no one said this was gonna be easy.
So: back home to finish the re-reassmbly (less a headstock screw and a rear left reflector). I get it all back together, and I'm re-threading and spinning the mirrors back onto the bike when the right one stops at an odd angle; I loosen it, adjust the nut, and try tightening it the correct angle when "snup!." Tragedy #4. Off snaps the mirror, with the broken, threaded end deeply embedded in the steering column.

Anyone know a good place to mail-order parts for an aging GT200? I tried Vespa Centrale in Hartford but couldn't get 'em on the phone.
But the clock's working like a charm. SUCCESS!