Congratulations on getting your Grandfather's tractor.
Some will say you don't need them but having the operator's and service manuals will help you with your planned repairs. If your grandfather had them and you can get them, great. The manuals along with questions on forums will get you through most repairs. AGCO has the Parts catalogs free to view on line. Go to https://www.agcopartsbooks.com/partsbooksn/login.aspx?region=northamerica
. Sign in as a Guest User. Enter D15 in the model search and your parts catalog will come up. It will let you see the parts related to each of the areas you plan to work on. They sell the manuals (service, operators, and parts if you want a copy vs using the one on line) at https://www.agcopubs.com/
. You can find used and reprints on eBay. The store for this site sells some manuals. Get OEM or quality aftermarket reprints of OEM. I have some, but I am not a fan of the I&T manuals, personal preference.
Does the power director lever have any resistance when you move it? You say it is stuck in high so I take that to mean it still drives. If lever moves freely it indicates to me that something has come unhooked or has worn out and failed in the shifting mechanism. You can see inside that compartment by pulling the side cover after draining the oil from it. The parts book breakdown shows it as the forward reverse shuttle with the parts for the power direct called out in the text. The shuttle shifting lever is on the opposite side of the tractor but the parts are similar so you will see what is in there.