TOPIC:

Can you put a different Kawasaki engine in the KZ440 frame and have it bolt right in?

Some KZ400s have the same crankcase, so I assume a KZ400. But you probably want more power, so no. 

The 440 models have variations in the frame dimensions. Kawasaki dealt with this by using different engine-to-frame spacers and fittings between models to make them hold a 440 engine. 
I believe the 400 twin will fit, with the correct spacers and fittings. You will need to check that it will in fact fit (I can't be sure), and then get the right spacers and fittings or make them yourself.
750 twin? Don't know.
250 twin? Maybe, if that's what you want.
Someone once told me (when selling a bunch of old Kawasaki frames and engines) that the 400/4 from Japan will fit a 440 frame. I'm not sure about this, because I would have thought the 400/4 is just a sleeved 550 engine. I don't think I've seen a 400/4 in the flesh.
If you see the engine you want in the flesh and take measurements, it should be clear.

The 400 B engine and 400 H will bolt right in, at least it did in a KZ440 A2 frame in my case – courtesy of the PO.
The only slight modification being the clutch lever cover on the left hand side needing a bit of trimming in its bottom or top left corner if I remember well.
The 400H has the same airbox as the 440 so you will just need a pair of Keihin CV32 carb in order to run it, or tweaking the CV34 you already have.

With the proper spacers and fittings, the 400 twins should fit. To ensure that it will fit, you must check. I will pm soon.