How do I fix this custom r15 rig to work with default animations?

I’m in a group of 3 working on a small game project. One of our devs found a free to use clown rig that we wanted to use as the ‘monster’ for our project. We wanted to make the clown rig hold tools and play R15 animations, so our animator got the model rigged and for the most part the rig works fine. However, for some reason the rig plays animations incorrectly.

Screenshot_651
Screenshot_652

The clown is supposed to extend the arm directly perpendicular to the body and hold the bat directly up, like a normal roblox character holds it up, but he’s not doing it

We need the rig to hold animations exactly how default R15 characters would, so that we can interchange animations between normal players, the clown, and other possible monsters we will add to the project. How can we fix this?