Thanks for the great tips! I did consider driving out the lower gear pin but instead followed Duane's well explained tips and was able to get the trigger wheel off with no damage to it.

I had soaked the heavily rusted center shaft for about 3 days in PB Blaster first. I think that what Duane explained and what is not fully understood by others who have failed at this is: you cannot force upward on the shiny metal that forms the 4-bladed wheel itself.

You must get beneath the thin metal, near the shaft, and position the 2 screwdrivers underneath the much heavier metal that the trigger wheel is attached to. You can see the correct place with a strong magnifying glass and good light while looking through the vanes. It is right above a second circlip on the center shaft.

Turning the shaft 90 deg. every now and then seemed to help, too. I also took my time with it - it would be easy to slip and perhaps do irrepairable damage to the wheel.
Jim