Hey,
If your gen. head is old the first thing I would do is inspect the brush assembly. If it looks ok then find another means of test spinning up the gen. head. and check for low voltage on the input side of the field windings. Then check the output side of the head. Many times an old head needs the armature stoned to remove rust and oil that impede the electrical process. A very fine emery cloth to remove the small layer of rust usually does the job. Drop me a PM and I will be glad to call you to help you. I been an electrician for some 30+ years so I've picked up a few tricks here and there.