You mean the whole top part that holds the barrel? I don’t think so, the HW45 releases the lever by pulling the hammer. The HW75 uses the hammer to strike the valve, so has a button to the left side of the hammer to release the catch. You might be able to use a HW75 top receiver on the HW45 but it will have an unused slot next to the hammer. You won’t be able to use a HW45 receiver on the HW75 as you won’t have a catch to hold the gun closed against the air pressure.
“We are too much accustomed to attribute to a single cause that which is the product of several, and the majority of our controversies come from that.” - Marcus Aurelius