The timing on the ZX750 is an analog, electronic advance system. It's determined by the shape of the ignition rotor and the thresholds set inside the box.
Is the ignition timing advance, between 750 and 900, different (other than static idle timing)?
If so, and you want to keep the 750 timing, you will have to compare some things.
First, is the shape of the rotor exactly the same between the 900 and 750? By that, I mean the base radius of the rotor and the rectangular tooth width and length. (If they have the same part number, they are probably the same.)
If the rotor is different, then hopefully the timing is different due to the rotor being different, and the box internals are the same (even though the boxes may have a different part number, due to different length wires or different connectors or different mounting holes etc). In this case, you'd have to try it and see.
If the rotors are the same then the timing is set by the box internals. So you would not be able to swap boxes without some modifications, unless you just want to use the 900 timing on the 750.
(I'm talking about ignition timing, not the valve timing, of course.)
Also, you could look into the Zx550A gpz550. They used a system like the Zx750.
