Welcome back to the hobby and to RCU.

Matching a missing screw or bolt can be tough. Since you have an identical one, bring it to to your local hardware store and see if you can find a match. Lowes especially has a pretty good variety of small rc-friendly nuts and bolts.

It would not be so bad if you had to replace the part. You could also try out one of the internet vendors. I have been very happy with www.rtlfasteners.com.

Good luck.