Hi Lori and welcome. That's one of the problems with old Sears routers is that parts are not available. It was made by Ryobi (the 315. at the start of the model number tells us that). That's called a rack and pinion adjustment. You might find an old one with a good gear. The gears didn't fail very often according to my observations. I think someone tried forcing it while the base was still clamped tight. There is also a chance that this gear and rack were available on other routers too since that might have been a part that Ryobi sourced out to another company. You would need to compare yours to the other to see if they mesh.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.