Hey @ThePortalo! Welcome to the forums!
One thing you could do: Have some variable Integers on your GameInstance labeled “Headshot Count”, and “Shots fired”.
When the player shoots, use "Get GameInstance → cast to GameInstance(whatever your gameinstance is), Set Shots Fired to “Shots Fired + 1”.
When the player gets a head shot, wherever the code is that confirms that, use "Get GameInstance → cast to GameInstance(whatever your gameinstance is), Set Headshot Count to “*Headshot Count + 1”.
Finally, you can use Get GameInstance + cast to Game Instance to get those variables, so you could do “Headshot Count / Shots fired” to get your percentage or any other stats that could use those numbers! Just make sure to use division, not the “%” node, as that means “Modulo”, not percent. 
Hope that helps!