sorry for seeing this question so late.
My reply is to grow out seedling white sapote trees, I did. I used the McDill fruit and the yellow sapote fruits from my local arboretum when they sold some fruits.
Once the tree is established, like 6-7 ft tall, they will be plenty of branches, then you can graft on most of the branches. My seedling McDill tree grew to 18 ft in 7 yrs, then I topped it and used all the branches for grafting and left a few big ones alone for seedling fruit. Today that tree is gone, but I still have the scionwood of the seedling McDill that fruited so it should fruit for me again soon. All of my seedling trees planted have flowers now so I should be able to taste the new yellow sapote fruit as well. I restarted my collection and now I back up to 27 varieties.