So I like to provide conclusions to my threads when I can.
First heater - Modine is now up and running.
Problem was the heater valve, replaced unit with an intermittent pilot unit (the type with spark ignition to fire up the pilot to then light the burner.
Benefit to this route is that 1 - my standing pilot won't blow out. And 2 - I should use less gas yearly than with a standing pilot, and I eliminated an ignition source for when I'm using volatile chemicals.....
Second heater is still resistant. Very similar symptoms, but the crack in the Heat Exchanger is troubling. So I may not convert the gas valve on that, but it gives me some time to do research. Almost considering some form of solar - evacuation tubes perhaps to provide day time heating, then I'll still have gas to maintain at night. On separate thermostats I can use gas to maintain minimum night temps, and then set the solar to bring the temps way up... things to consider.