Cool. If it was me, I'd start by trying to hook up with who ever told your grandma that it was a $75 fix and see if they can tell you what it was that it needed. Do a little trouble shooting of your own to see what it might need. Making sure it runs is the next good place to turn to. You wouldn't want to sink a lot of money into the drive train first if it won't run. Then put stx38 in the model number on the John Deere parts catalog to get the drop down menu of choices. There was a black deck and a yellow deck version of that tractor. Look at the diagrams and parts lists and see if you can find the thing that'll get you fixed and going again. Use the part number to find yourself the best deal, or share with your parts source.
And have fun with it.:thumbsup: