Yeah, the RG321 is fixed bridge.
Anyway, nak, yes, just open up the cavity behind, there's only 1.
You should see a neat bunch of wires. Remember, be careful and unplug the guitar from the amp before doing anything. Now, do note, if you damage something, it ain't going to cost you hundreds, likely a few bucks should you knock some wiring loose.
Now, see if the 9v batt can fit in. it can be on top of the electronics or inbetween. Try to get an area that is slightly larger than a 9v.
Report please!