John Gumbs
Unit installed in 2002. Had a voltage regulator failure in 2018 and given to a friend. Installed unit with new regulator and started - ran OK. Set voltage and Hz - ran fine. Had trouble with transfer switch transferring back to line power during trial runs. Next day, tried to start and got a "smoke test" from control board - pin 13 common ground. Unit would not crank nor run. Friend called a friend who works with Generac units and the control board was replaced with one whose p/n is 10000036086A (K1921 I F1 on circuit board). Unit would crank but no spark. I contacted Ziller thinking board was incorrect for the year/s/n. Ziller supplied a replacement board 0d86150SRV based on year and s/n of unit. Unit cranked but no spark on either cylinder. I doubt that both magnetos failed at exactly the same time so I am looking for other reasons the unit cranks but will not fire. Checked connections, grounds, wiring. Thinking I am missing something (input) to circuit board or similar
Anybody have any further troubleshooting ideas to get this old girl running again?
Try disconnecting the #18 wire to the coils and see if you have spark.  If you do then I would look for a short in the harness.  On those units the 18 wire should be open to ground except when in the off position.  If you don't have spark then your coils are bad or the coil harness has a short.