My first option was to cut out the enclosure a bit more to get a normal sized microwave in there, but a woodworking friend pointed out it would look funny because the hole would extend all the way up to the bottom of the cabinet door above it. The wife was not pleased with that choice either.
My next option was to repair the one we have. After researching microwave ovens, the troubleshooting guides pointed to the overvoltage diode and the magnetron as the likely suspects for the symptoms of this oven. I disassembled the oven and took out the items. The diode was open in both directions so I immediately suspected it. I checked the magnetron for continuity and the leads had 0.000 ohms resistance between them. Hmm, that looks bad too. The high voltage capacitor seemed to be charging and discharging when I put the multimeter leads on it. I ordered the parts with next day delivery and the total was more than $200. Ugh...
When the new parts arrived I immediately ohms checked them and found the very same results as the ones I took out -- the diode was open in both directions and the magnetron was shorted. This time I was not pleased!
Facing no other options I went ahead and installed the diode first and heated up a cup of water for a minute. Still cold. Next I installed the magnetron and held my breath while I heated up the cup of water. After half a minute I noticed steam inside! When I took out the cup after a minute it was hot. I showed the cup to my wife and this time she was pleased. The microwave oven is back in the kitchen cabinet and working fine. Warming up leftovers in a pan or heating up the leftover morning coffee were good incentives to get this resolved.
I'm still baffled as to why the diode has no continuity. Is it because my 9-volt multimeter doesn't have enough current to get through the junction? I don't understand the magnetron properties so perhaps ohms checking it was a useless test.