My 26 has not been started in over 45 years. I always started it on battery and switched to mag. No starter on it. If I oil the cylinders, leave the plugs out and have a rear wheel jacked up, would I be able to get a voltage reading on the mag post while cranking?
Would it be AC and could it be read on a digital meter?
Yes it is AC and you will get a reading if you use an ANALOG meter. Digital meters do not do well.
I would hook up a battery and try to start the engine, then with the engine running on battery test the magneto. It is unlikely that you will get very much out of the magneto by cranking.
Maybe you could hook it up to an oscilloscope and record the wave form on something like an EKG for later viewing. But even then you would need to keep a constant cranking speed for a given time.
A magneto voltage meter such as this is available from either Snyders or Langs.
In my view your going about this the wrong way.
First get the car running on battery. Then get a credible magneto measuring device (St Louis Magneto Tester) and test the magneto.
Other tools like shown above are not up to the job of giving you a real picture of the health of your magneto. If you need help contact me.