Not related to the missing piece as such, but a word of advice.
Get some puncture “proof” tyres – changing a rear wheel flat on a geared Roadrat is (for me anyway) a royal pain in the ass. You end up with the rear mech and what have you flapping around after you’ve removed the wheel and then you’ve got to get it all back on again. I never figured out how to do it easily in the 5 years I had one, changed it for a Kaffenback in the end.
Don’t forget the rear wheel has to come out backwards when you need to remove it, if you’re running full length guards, you need to mount them accordingly.