In that file you will find all the modded/upgraded vehicles that spawn around Los Santos.
There are various sections headed:
that define the vehicles that spawn at certain locations. It should be pretty easy to figure out which one you want to replace. Search for '<Name>RUINER</Name>' etc to find the vehicle you want to replace & then change it's name, body colour & upgrades to the values for your modded Coquette.
Note: There are 4 modded Ruiners in there. Use the bodycolour codes: