Sell what in kit form exactly?
You can buy the parts you need from gorillamaxx themselves as spares, just look in their spares section under the G3 chassis- you will need the skid and rear upper plate. RCM doesnt sell the spare parts, just the whole chassis. I honestly wouldnt worry about needing a Ti skid on the rear, though a trimmed down item would fit over the top of the gorillamaxx ones nicely (just use the 4 holes that attach it to the underside of the bulks, and trim off the rest if you use a normal length skid). The aluminium skids of the gmaxx chassis are very short, not like a normal skid that is inclined to bend if you hit something- take a look at the photos of my truck to get a better idea of what you're dealing with:
http://www.robotwars.00server.com/gmaxx.html