Frames are mild steel.
Somewhere I've seen a build thread on adapting a single-shock rear suspension, perhaps off an early CBR600 Hurricane. Happy searching...whether you find it here or somewhere else in the web-o-sphere, it sounds doable and fun (although for a "traditional" cafe bike....dual shocks would be equally fine).
Only the SOHC CB750 was dry-sump with a separate oil tank. CB550, CB500 and CB400 were wet-sump, so no oil tank to fool with.
KZ as in Kawasaki?
Tuneups are dirt cheap if you learn how to do it yourself. On the early Honda fours, you adjust the valves with a "screw and locknut" method, and if it still has the original points ignition, you can file and regap the points, then set the timing.
Have fun...welcome to the fold!
Kevin