That .18 is a nice engine as well, though I would want a non-pullstart since I use a starter box. I don't know how everything holds up with the power of a .18.

I got to thinking if you are replacing a stock motor then you probably have a side exhaust? Because the Picco I told you about it a rear exhaust and if you don't have one you will need a new manifold, springs and pipe.

Are you looking for a side exhaust or rear? Looks like that OS .18 is a rear exhaust also.