Problem solved! As I suspected, it was the toggle switch that the previous owner had substituted for an ignition switch when the old key switch went bad. It had worked fine, and was beefy enough that after ohm meter tests showed the switch was functioning properly (with an open circuit when off, and zero ohms when on) I was not all that suspicious until after I had double checked everything else. I knew that something was obviously robbing amperage, so first thing I did this morning was to disconnect the negative cable end from the battery, and connected my digital multimeter to see if any current was flowing between the cable and the battery post. There should be NO current flowing when the ignition switch is off and no devices are in use. Sure enough, this test showed a draw of 5 amperes. I removed the ignition switch and its wires from the circuit, reconnected the battery terminal,and then ran a wire directly to the coil negative terminal from the Load connector of the voltage regulator, thus bypassing the switch altogether. The wire end, as it touched against the coil's negative terminal, gave off a visible arc, so I knew the switch had been the culprit. Sure enough, the engine started up immediately when I turned gas flow on and engaged the starter. Later on I went to the local auto parts store and purchased a well built key switch that I installed upon returning home. Also purchased a new toggle switch for lights, as I'd like to have some running lights this winter when plowing snow at night. The previous owner had removed and saved one of the old headlights, and one rear lamp. I haven't tested them yet to see if they are any good, but would like to have a pair both front and rear, so will be looking to see what I can find. Any suggestions on a source for original lamp fixtures? If so, thanks in advance.
By the way, that defective toggle switch was marked rated at 30 amps, which would seem more than adequate, but was definitely not the original light switch as it is marked "Made in China." Nearly all electrical components sold in auto parts stores nowadays are cheaply manufactured in China. Sad, but true.