My 1991 has no appreciable lash, but it only has 19000 miles. With the bike on the center stand and in first gear, rock the rear wheel back and forth. I'm not sure what the normal play is but listen for clunks or clanks.
Likely causes have mostly been mentioned above...rubber cush drive units are a likely suspect. Check out the condition of the drive splines that fit into the rear wheel hub. If a PO wasn't careful about lubing these at every tire change with moly grease, they can wear. Could be the spline in front of the final drive that connects it with the final drive shaft...lack of grease can result in wear and play/clunking. If a clunk is coming from the right side of the swing arm, close to the engine, could be the upper upper splines or the universal joint have play. Just some things to consider. When I brought my 1991 home last August, I pulled all this apart, checked, cleaned, and re-greased everything...it had been sitting for most of the previous 20 years and I was concerned about the effects of time.
If you ever need, there IS a Youtube video showing how to shim the bevel gears in the final drive. Not likely that you would need to do that.