I used the Kyosho motor mount on my matrix. It works if you use the Kyosho center diff mount on the other half also (kyosho part number IF131).
http://www.bmainhobbies.com/product_...45063ca779347e
http://www.bmainhobbies.com/popup_image.php?pID=354
That part number is for both halves of the platcic Kyosho mount. One of the half has a deeper bearing counterbore than the other. If you look at the picture I linked you can kind of see that one is deeper than the other. Use the part the half that has the shallower counter bore.
The cen diff is actually about a .5mm shorter from bearing to bearing than the kyosho so it will have a little play. I used some diff shims I had around to take up the extra play. Although the play isnt very much and you probably wouldnt even have to worry about it.
Hope that helps.