My son's ARRMA Raider buggy burned out its stock motor after a couple weeks worth of use and we brought it in to get it replaced. The local hobby shop (ABC RC & Hobbies) didn't have a stock 15T ARRMA motor around so they suggested we try the Traxxas 12T Titan motor. While not a perfect fit--the 12T motor just slightly larger, it did fit with VERY minimal finagling. The biggest modification you need to make is to Dremel off about 1MM of the new motor's output shaft as its just a tad too long to fit under the plastic cover.
After installing it, it was time to take it for its first test. The 12T motor seems to provide a solid upgrade over the 15T motor as the Raider's speed is noticeably greater while ripping through the grass and sand with greater ease. Can't recommend this upgrade more. Easy to do, basically a plug and play operation and impressive results.