Hello!
I have a really reliable standalone CO2 detector with power and alarm led indicator. IS it possible to cut off the leds and connect it instead to a raspberry to monitor power and alarm state of the detector? 
Yes. Look at replace LED with an optocoupler.
Or replace LED with relay that trip high/low RasPi gpio.
There is also a camera component that can do this using a camera I believe
EDIT
CO2 or CO detector?
I was gonna say there are zwave CO/smoke detector as well which may be easier.