As i told you a 18 would not be legal for sanctioned races ,and the other thing is that there is allmost no good 12 with a pull start ,i would really go for a starter box that would give you much more choices in engines .
If you really want a 18 you would have to gear up your clutch bell to at least 3 teeth to not have something uncontrolable at low speed's but still you will get something that becomes stupid fast at high speeds for a real track .
After all it's your choice ,no one can decide for you and whatever you take you will have to live with it ...