Next surprise: I have a nice Kobalt 3/8" drive socket set (metric and SAE), and I picked up a nice Craftsman 1/4" drive metric socket set on my last trip to Sears for $20, and I also have a decent collection of 1/2" drive SAE sockets (normal, impact, and deep impact). However, I only had a 1/2" drive torque wrench. Rather than buying some 1/2" drive metric sockets, I bought a 3/8" drive torque wrench at Sears. I figure most of the bolts on the bike are 14 mm or smaller, and shouldn't require more than 80 ft-pounds of torque, so I should be pretty safe.
Luckily, on my last trip to Lowes, I picked up a set of strap wrenches: A big rubber strap on a handle. I used the larger strap wrench to hold the alternator rotor in place while I torqued down the bolt that holds it on the crank. It occurred to me that the reason this problem occurred in the first place could have been due to that particular bolt being loose, allowing the rotor to hammer against the Woodruff key until it failed.
Replaced the gasket ($4) and generator cover. Fortunately I still had some Permatex Gasket Adhesive around, so I should have a pretty good seal. I was able to turn over the engine with the starter motor; it feels good to actually accomplish something.
Next up: Valve clearance, change oil and coolant, install spark plugs and ignition system, re-attach and synchronize carburetors. Then, new exhaust system...