FPS Arms replacement?

I have not done much with UE4, one Udemy course. I would like to have a go at a FPS but all tuts I see only deal with those mannequin arms. How easy is it to replace the standard UE4 arms with custom ones?

As long as the arms use the unreal rig, you can retarget the animations to the custom arms.

There are some arms in the marketplace, if you want to pay.

You can retarget to a custom rig, but it’s just easier to have an unreal rig.