What's the best approach to swapping out this alarm for a new ring alarm?

[DanMcLiverpool]

I have the following alarm system, but I've just purchased a new Ring alarm bundle that I want to swap it for.

Couple of reasons why I haven't just ripped this out to start over with my new one:

1. The original alarm keypad by my door has a power cable I'd like to use for the new Ring keypad (which I assume is coming from this box)
2. If possible, I'd like to use the outside siren plugged into this G4 box (might not be possible?). The Ring comes with it's own siren, but it's an indoor one.

What's the best way to go about this?

Also worth noting in the first pic I've pulled the fuse out of this from the box underneath, and there's no siren, so I assume the battery's broke in it?

 

[DanMcLiverpool]

I'm thinking for number 2 I can achieve that other ways, using IFTTT with a smart switch or something (i.e. IF Ring alarm goes off, switch siren on), so don't worry too much about keeping that siren wiring in place.

My main concern is with the power to the keypad etc.