Since it is Lucas someone else needs to verify this:
Disconnect FIELD wire from generator. Connect a seperate wire to the field post of generator and ground the other end of same wire to engine block or generator housing. That should give wide open generator out put if the generator is good. My To-20 has been running that way for 18 years with no problems. NOT the way it SHOULD be but it DOES work. It"s a good test procedure for generator. Most one wire alternator systems work the same way.