The heat from the battery plus the heat from running the engine, it will not last.

A lot of the Helicopter croud have remote plugs. Some of their engines are mounted inverted, and if you put the Ni-Starter on it, the landing gear will not set on the ground.

http://www3.towerhobbies.com/cgi-bin...?&I=LXBC35&P=7

Try this, you mount the plug head where it is convenient, like where you have already cut the cowl around the exhaust.