You'll need to pull the rear wheel off & pull the hub. The rim is not threaded, it has spots inside where nuts fall into place (plus you will want to hold the nuts in place while you start threading your bolt in. There should be like a ring (looks like a head gasket almost) that sits up against the sprocket that once you tighten the bolts down you flatten the edges of the ring around the nuts to stop them from vibrating loose. If you dont have the ring anymore i would recommend you put lock nuts inside the hub.
Now might be a good time to replace the shaft seal sitting behind the engine sprocket. They are known to leak... evidence of this is oil flung all over the rear wheel.
1985 tomos is a good bike. It had the better made tomos clutches inside however your A3 engine is likely running the stock encarwi carb. I would suggest you track down a later model tomos A3 dellorto intake and a dellorto SHA 14/12 carb. Your bike will run better and faster with that "upgrade."