A mamba brushless motor made for 1/10th will not power a 1/8th scale very long without overheating. You need to get a brushless motor more suitable for 1/8th scales. For example, a feigao 8xl motor etc. I am not a conversion expert, but i am sure some of those guys will reply on what you need exactly. You can also check
www.rc-monster.com, or read the 1/8th conversion thread on here.