You will have to do some leg work to find where the voltage drop is. This is very common with the old G series. Could be one or more of the following:
bad connection in the harness somewhere.
bad combination box
bad PTO switch
bad brake switch
bad ignition switch
or...worst case, bad wiring.
Have seen all of them at one point or another, though, ignition switches and pto and brake switches are the most common switches to die.
The harness connectors are not sealed, so water gets into them and corrodes the terminals. I had to take all of the connectors apart on my G1900S and clean EVERY terminal; then put them back together with dielectric grease, which is still just temporary, but it beats leaving them dry. The voltage regulator plug was actually burned up and could not be saved, so I took the opportunity to install a real alternator.
There is also a way to just rig it. One such way is to install a 12v relay in the solenoid wire. This way the load is taken off of the starting circuit and placed on the relay, which should give you a full 12v to the starter solenoid. I use a John Deere part number AM107421 relay kit for this, bolts right onto Kubota's too.
