A test with an ohm meter on the coil secondary should tell the coil health. Not sure what the actual reading would be on an A50 coil but I would assume from testing many Johnson coils that the value would be 3000 to 8000 ohms.

Johnson coils unless physically damaged have withstood the test of time and are almost always good.

I believe the Johnson A-50 is the first generation of Johnson’s alternate firing twins.  I also believe that they used two coils, one for each cylinder up until the AT’s and LT’s which used one coil and grounding out the side for spark that is not ready to fire.  So, the A-50 has two coils.  If you are replacing the wires, be careful.  Cut the old wire off of the coil leaving about 1/4″ or so tail so you can solder the new wire directly on the tail, NOT onto the coil.  Use minimal heat meaning just enough to melt the solder and stick to the tail.  After you solder the new wire onto each coil, then test the secondary ohms.  You should get a reading of say between 1500 ohms to say 5000 ohms.  If you get no reading it means the secondary circuit is open and the coil is shot.  I hope this helps and I am sure if I am found incorrect, I will be beaten senseless with a dead trout.