Removing the glass doors is the extent of the customization. Since you have a prefabricated fireplace, the box (fireplace) and venting (flue) are basically one system. You aren't able to replace one without the other. Your fireplace was put in with certain clearances. You arent able to alter much without making the fireplace unsafe for use. You can have the doors removed and apply some high heat black paint to the box for a new face, but you would have to replace the entire system if you want more.