If it rotates, balance it. Clutch drum, engine sprocket, wheels for sure. I have an HRD 1949 Comet. When I bought it, it had a 21 in front wheel and a 450 x 21 tire. I drove it about a mile, brought it home and took it apart. Changed out the front rim and tire, and trued it within 10 or 15 thou, and cut 2in off each end of the very wide handle bars, which were the only way you could turn that big front tire. The back tire/rim was the tell tale tho' as at about 50 mph it bounced right off the ground!
I removed the tire and trued the rim from about 1/4 in out down to the 10 or 15 thou. I then put it on the bench and it took off. I know my bench might be a bit off but not that much. Experiment showed a very out of balance rim/brake/hub assembly. I put the tire back on and then balanced the lot. Big amount of weights added!
Now a bike that shook, vibrated, was a pig to ride, is turned into a reasonably nice ride. Now just to replace that clutch with a modern one. Maybe tomorrow.
Look for the easy ones first, and if vibration is still an issue, then the motor needs to come apart!
If it rotates, balance it.