Thank you for all the suggestions. I got it running. It was the ground as a few of you thought. I still have a couple of questions. I tried running a jumper cable from the negative of the battery to the engine compartment. The engine looks painted so I didn't try that. Would that have still worked? Instead, I used the metal latch that's bolted to the frame. It is steel with a gold finish that I guessed would conduct electricity. The engine started! I found where the black wire went to the frame. It all looked fairly corroded. I replaced the ground wire and cleaned everything up. It now starts fine.
Another question: What causes the white powder corrosion? Is it a sign something else is wrong? Will it be less now that I've established a better ground? If anyone reads this and knows how to access the side of the engine facing forward; I would still like to know. I'm pretty sure it's not under the seat, although that's possible, a lot of stuff would have to be removed. On the left side, there's a panel that the fuel filler comes through. Only three bolts appear to hold that panel on to the frame. I can see the inside of it from the back engine compartment hatch. My only hesitation is the fuel filler. There's a rubber seal between it and the panel. Do those usually come off and go back on easily?