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Motoring isn't always the best way to get residual magnetism in the field pole shoes. The best way to do it is to isolate the 3rd or movable brush from the commutator and jumper direct to the field wire on that brush or disconnect the wire from the brush. The ground on the other end of the field also needs to be clean and tight. Tom Wessenberg always said to check the field coils by installing an ammeter in between the power source and the coil and then apply power to the filed coil and see how much current it will draw. The amount depends on the configuration of the dynamo, whether a 5 brush or a 3-brush type.
This link has a post by Tom on current draw for all three types of fields. He used a battery charger with it's own amp meter to test. This will polarize the pole shoes.
https://www.fordbarn.com/forum/showthread.php?p=1517922
There are plenty of other problems that can happen so a person has to growl and continuity test the armature, then inspect the brush plate for any signs of loose connections (especially the 5 brush units). Make absolutely sure that armature and field coils are insulated well even if they have to be resprayed or wrapped. Check the air gap between the armature and field pole shoes. They should be pretty close. If the gap is more than .010" then the magnetic gauss or field strength becomes less effective. I'm not sure on maximum limits for the powerhouse but they should be close as possible and the shoe thickness is what regulates this.
An electromechanical cut out relay is set to just above normal battery voltage for it to come on line. It is very important that it has a good connection through the points. It is also very important that that the battery be fully charged and in good condition since that is what regulates the system voltage in any 3-brush or 5-brush system.