what would be the best way to make this rock look complete or whole
You simply can't. You don't have the geometry information of that area so there is no way to display said area. However if you're only concerned with the result looking less ugly there is a number of things you can try:
- Rotate or obscure the rock in your composition so that the cut places don't show in the render.
- Add the missing geometry by hand. You decide how much detail/ work you're willing to invest.
- Use the surfaces you already have in order to composite a new one.
- Use AI-magic to create the missing surface (although I have no idea how to pull that of).