If you really want a 18 ,here is a good one (i have it without the pull start ) ,reliable and easy to tune .
http://www3.towerhobbies.com/cgi-bin...?&I=LXHLV9&P=7
You will need a new manifold for rear exhaust :
http://www3.towerhobbies.com/cgi-bin...&I=LXTWH5&P=SM
You will also have to get a adapter from a NTC3 in order to put on your clutch bell and a cut off nut to cut the cranqshaft to the rigt length .
http://www3.towerhobbies.com/cgi-bin...?&I=LX3382&P=Z
http://www3.towerhobbies.com/cgi-bin...?&I=LXBMD5&P=Z
With all this you would have a rocket with the benefit of an easy to start and tune engine .