In about 2005, our 4240 needed an overhaul, and we had a kid that worked for us that was going into the John Deere Tech program in MAdison, WI.
The 4240 was his guinea pig. Anyway, the head had several cracks, and so we replaced it with a 4440 head and turbo system. I don't recall if the rods and pistons had been upgraded to 4440 specs... I could check.
Using the 4440 head lowered compression a little, rather than just bolting on the turbo t othe natural head.
We left on the rotary pump, and didn't touch it from factory specs. This resulted in a 130 hp tractor... that seems to get a fair amount more work out of the same amount of fuel.
The muffler hole had to be cut out a little bit, maybe a 1/4 inch to the left to clear the muffler.
The whole thing turned out rather nicely, and while it's still not a 4440, its a lot more tractor than the original.