Your test doesn't give much info unless you can tell us how many amps it is making when you full field it. Also need to know what speed the alternator is turning (not the engine RPM).
That alternator - when full fielded - should easily put out 30 amps @ 15 volts when spinning at 3000 RPM. I don't know off hand what your crank-pulley to alternator-pulley ratio is. Most likely it's 1.5 to 1 ratio (most Deere farm tractors with alternators are). So, for the alternator to spin at 3000 RPM, the engine must be turning at 1920 RPM.
The Delco SI series e.g the 10SI, 12SI, and 18SI are nowhere near as rugged as the old Motorolas. Weak diode trios, no ball-bearings in back, etc. They ARE however, cheap and common and easy to work on without a soldering iron. Deere made a few new tractors with Delcos and they did badly with farm and forestry use.
Full ball bearing Hitachis, or Delco CS alternators are much more rugged then the Delco SIs.
If a 35 amp alternator suits your needs and you want to keep the Motorola - make sure the slip-rings where the brushes ride have not be destroyed by dust. If they are OK, chances are all you need is a diode or two. The isolation diode unbolts off the back and you can change one in 5 minutes. A new one at NAPA is $10.
New regulators are around for less then $20. Prestolite 8RB2007, WAI M5146, etc.
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