Being able to use CAD has huge advantages. I was able to track down XS650 frame specs and from there, able to design an entirely new setup for my old 650. I was able to determine determine swingarm lengths, and use the frame shock mounts to finalize where to weld the shock mounts on the swingarm. And with that new swingarm, determine the correct Rake and trail for that wheelbase for optimum performance.
A helpful trick is...Try to find a modern bike that will fit your riding and handling needs. Than mimic those dimensions if possible. As another member mentioned, times have drastically changed in bike design, and attempting to get 20005 GSXR handling from a 70's bike is near impossible. BUT.... getting things safely close, makes a huge difference in feel.