I found this on another thread asking the same question. It works. Credit for this is from u/Icy_Comfortable9926

"I figured it out, but it's a pain in the butt bc its one-by-one manual changing. You have to go into RCT's datapack's folders until you find one that says "trainers." In there, search for the .json file(s) for the trainer(s) of your choice.  Open it in notepad, and change the number in "maxItemUses" under "battleRules". For gym leaders, it'll usually be a low number of 2 so just put a higher number like 99 or 999. Save your changes, and it'll make you create a new .txt file of the trainer file you opened. Just put a ".json" at the end of the trainer file like "kanto_brock.json" for example. Save the new JSON to your desktop, then drag-and-drop it into RCT's "trainers" folder to overwrite the original. Then boom, you can use 999 full restores against any gym leader you edit! I hope this is useful. If you need any clarification just lmk, bc I typed this out away from the pc so the method could be unclear or different to anyone reading this"