Thanks mechanic mark and rivets for your replies.
The machine has 2 starter solenoids in the starting circuit. The primary solenoid which closes the circuit to power the starter is mounted on the starter (typical of an automotive starter) and isn't serviceable/replaceable to the best of my knowledge. The second solenoid supplies power to the primary solenoid on the starter and is replaceable and inexpensive ($10 - $15 depending on source). I've ordered it and plan on installing this weekend if it arrives by then. I can test to determine if it's the problem prior to the new relay arriving by starting the engine and then disconnecting the lead from the second solenoid to the primary solenoid.
I'm hoping the problem is due to the second solenoid as removing the starter from the engine appears to be a real job. The mounting bolts are fed through the rear of the block so I would have to remove a bunch of items to get to them.
Again, thanks for your replies and I'll follow-up once I have a better idea of the source of the problem.