Rear wheel change

I just recently bought a kz900 someone had changed to a 16” wheel.  If I go back to original 18” can I use the same brake hub assembly and back sprocket   

Yes you can.  When I first got my Z1A the PO had changed the rear wheel to a 16".  I bought a used 18" wheel and everything fit .  He had also changed to the smaller front fender from an LTD, which I liked, so that stayed.
{apologies for the late reply !}

Just buy a new 18 inch rim and new spokes and have it laced and trued. Maybe that was your intent.