A couple of suggestions :
GSP Swallow EX
Ultra RC is re releasing the Quest (but I don't know when)
and my favorite

The excelleron by Sportsman Avaiation
http://www.globalhobby.com/public/gallery/127565.asp
I acquired a OS 108 for my Explorer/excelleron but chose to get a YS110 instead. I don't know enough about the St90 to comment but if that's a 2-stroke and lighter than the 108 it'd probably be a good choice.
Eddie