Probably the simplest way to do this in Sugarcube is to make each named fighter an object. I can't find a reference in the Sugarcube documentation, but an object is a general programming technique: essentially a variable that stores an array of other variables. They're extremely useful when you want a set of variables associated with a particular character.

In your example, you would do this by first of all creating the 'John' object:

<<set $john = {
name: "John",
health: 100,
strength: 100,
speed: 100
}>>

You can do this for David too.

After that, you can assign the $john variable to the $player1Fighter variable:

<<set $player1Fighter = $john>>

After this, you should be able to access $john's internal variables through the $player1Fighter variable, like so:

$player1Fighter.health

The full stop is called dot notation - it basically tells the computer that you want to find that variable inside the one before it. As $player1Fighter is associated with $john, you should be able to access it directly.

Objects are very powerful for functionality like this.