KK and Michael, thanks a lot! This part is indeed missing on my bike!
Do you perhaps know where could I get one? I was trying to find it on eBay, but no luck so far.
EDIT: just found this one, I believe it is the right one:http://www.ebay.com/itm/Kawasaki-KZ750-M1-CSR-Twin-1982-82-Chain-Guard-55020-1152-NOS-/271256614771?pt=Motorcycles_Parts_Accessories&hash=item3f28277773&vxp=mtr