A single-wire (self exciting) alternator is not a standard used on mainstream cars and trucks or tractors. Many places sell Delco SI or Delco clones from China brand new with one-wire regulators installed. You can buy a brand new, one-wire alternator right here at YT Tractor just as cheap as most other sellers.
Keep in mind - a Delco SI does not have the exact same footprint as the AJ12N Motorola alternator you have now. So a few minor mods are needed to install it. NOTE - the fan belt width. You likely have a 1/2" belt and most of these alternators come with 3/8" pulleys which will not work. Plus, a one-wire alternator has no hook-up for your "no charge" light.
YT tractor sells a new Delco clone from China with the wide pulley you need already installed, and set up for one wire.
I have a Delco SI on my Deere 300B but I used the OEM three-wire hookup to keep my warning light working. 3 wire also charges your battery more accurately then a 1 wire, not that it matters much on a tractor.
DB Electrical also sells them with free shipping and will install the correct pulley if you call and ask.
You can also take the pulley off your Motorola and just stick it on a Delco SI if wanted.