The 0.7 modulus 132T main gear is carefully machined out to fit the stock Trex 450 autorotation hub. 
Gears of this size are used on a number of 400 class helis like the Shogun and Revo-CP and are readily available on eBay.



This photo shows the modified left side frame required to have the pitch servo fit inside the narrow "doghouse".   This frame mod is detailed in the plans which are available from the AMA Plans Service.



This shows the motor mount mods required to use the smaller 132T main gear.  The slots are extended, and the screw bossed are partially milled away to give the motor greater travel.



Here are the assembled main mechanics.  Note that the servos are installed with their mounting lugs inside the frame to make a little more room - this is a very close fit.  The aluminum bearers are cut from 1/2" extruded aluminum angle from the hardware store.  I recommend swapping to longer screws for mounting the bearers (M2 X 16mm).  The lower frames are cut off flush with the bottom of the bearers.  


Here's the final arrangement of the mechanics.  The battery mount has been modified to a horizontal orientation in order to provide a mount for the 9-channel receiver and gyro above and the Phoenix 35 ESC and CC-BEC below.  That's a lot of hardware in a tight space, but it all fits, and the accessible gyro is a real plus.