Having just recently moved the power system from a Belt CP V2 to an EXI 450, I thought I'd share my experience. Namely, what additional parts I needed.
You will need to buy an 11 tooth pinion gear (for 2629 rpm approx according to
http://dhrc.rchomepage.com/calc.htm), and you will need shorter motor screws.
I belive the shaft on the Belt V2 motor is 2.3mm. The stock motor shaft isn't long enough with the pinion that it comes with, so get a pinion that will effectively extend the shaft and get the pinion teeth where you need them.
The motor screws from the Belt V2 are too long for the EXI frame.
I also had to use my dremel tool and cut the servo horn for the Belt V2 elevator servo. I'll post pictures soon. Basically, wherever I put it, it got in the way of something, so I just trimmed off the one side so it looked like the other two.
Keep in mind that since the motor mounts from the bottom on the EXI, (as opposed to the Belt) you will need to reverse the direction of you motor. I did this by switching the red and black wires.
I also had a little trouble with placement for the ESC. I have it in the front laying on the bottom tray, but the wire that goes back to the receiver (which I put in with the plugs slightly overhanging the back of the heli frame for easy access) is BARELY long enough to keep the ESC from touching the motor. Just make sure to use some double stick tape and tightly zip tie it so it can't move. You DON't want your motor to rub a hole in the capacitor on the ESC mid flight, I promise :-D
BTW, I have to credit Meanbaby for most of my setup, he helped me route all the wires and figure out where to put the ESC, receiver, etc.
I'll try to post picture of my servo wire routing too. Meanbaby had an excellent way of running them through the center of the frame and securing them with zip ties and double stick tape!
Hope this is helpful...
Chris