http://www2.towerhobbies.com/cgi-bin...?&I=LXKF97&P=0
http://www2.towerhobbies.com/cgi-bin...?&I=LXFMD5&P=0
http://www.towerhobbies.com/listings/cat-cat-g.html
If the first two are a little more than you wanted to spend, the 3rd link is for the whole list of the .40-.51 size engines at Tower.
http://www.kangkeusa.com/
These on the last link are good power for low money! Check them out too. I have a .56 from these guys & it screams.