If you just want to check the governor setting you can also get a machinists mechanical tach and hold it on the top or bottom of the crankshaft.
Does a dwell tach take the signal from your plug wire? If so you just need to divide the reading on the tach reading by two and you have the motor RPM. Ie a reading of 7200 means you are actually revving 3600. However I doubt the gauge goes that high so you will probably not be able to do it.
If it takes it from the coil wire (Lead) you need to multiply the RPM by 4. Ie a reading of 900 is 3600RPM.
If the dwell tach takes a reading from the negative side of the coil, you wont be able to use it.
Does it show my age when I dont know how to use a dwell meter