Drew,
This is what i did for the gear reduction:
I bought 2 sets of gear box from
http://www.jkaerotech.com/electric.htm. Each set came with 3
pinions for different ratios (2:1, 2.5:1, and 3:1). I also got two
new s400 motors from Tower Hobbies. I used the smallest pinion for
the motor, and the mid size one on the output shaft of reduction
box. I had to drill the pinion to fit the output shaft, which is
4mm. I marked, drilled, and bolted the reduction set to the gear
box using the two existing motor holes, with the small machine
screws and bolts (m2.5) from the hardware store. I did not need to
cut anything off the black plastic covers.
I used the pictures on Willy's site for references. It was very
helpful.
I did this about 2 months ago, but have not finished the kit yet. One thing you may want to do is to try to mount the right motor as close to the original location as possible (may have to cut the plastic cover a little for this), else you will have problem with the batteries fitting. I found that i got a little bit more room by not installing the steel plate for the electronics.