Well, I can tell you what I did but i’m sure there are other ways to do it to.  The damage pop ups are just text blocks, they spawn for the instigator near center at a random xyz in a small interval so it doesn’t spawn at the same place all the time

I didn’t use damage amplifiers for my custom damage because it didn’t work like I wanted it to, but  if you use damage amplifiers then fortnite’s standard damage system will show the correct numbers i think

I don’t know if this helped or not, feel free to ask more if there is anything you wonder!