So the best way would be to store the original values in a Dictionary. If you are using export variables to set the numbers, then in the _ready() function your would have to save them into the Dictionary. If it is just a couple values, then it would be easier to just use single variables, but if is more than 2 or 3, then a Dictionary is better.
extends Node2D
export var player_health = 100
var original_player_health
func _ready():
original_player_health = player_health
print("original_player_health = ", original_player_health)
player_health = 33
print("player_health = ", player_health)
reset_variables()
print("player_health = ", player_health)
func reset_variables():
player_health = original_player_health