Hi all, I have this Scorpion Alarm system which is quite old but still works apart from the fact that I have to for some reason regularly clean the board behind the keypad as the keys stop working. Also, one of the sensor cover just fell off today, broken clips so won't go back in. Battery was replacedcouple of years back and now won't work for some reason though I tested with multimeter and shows 12V. Also, the outdoor siren is old metal type and is mounted quite high on the front of the house, siren doesn't even work anymore and it needs to move or replaced as house getting painted soon and the rust etc is getting on the house near it. 

What are my most cost effective options? Can I just replace one sensor with any sensor and the outdoor siren and possibly the battery and move on or is it better to have a new alarm system? Would the installation cost be bit cheaper considering all the wiring is already done and can possibly be reused?

Here are some pics, sorry some are rotated and I don't know how to fix it. Original pics aren't rotated.