The .46-Ax is on the high performance side so I would say at least a .70-.72 4-stroke.
Some options are:
http://www3.towerhobbies.com/cgi-bin...?&I=LXHSL2&P=0
http://www3.towerhobbies.com/cgi-bin...?&I=LXEXP5&P=0
http://www3.towerhobbies.com/cgi-bin...?&I=LXGRP4&P=0
http://www.horizonhobby.com/Products...ProdID=SAIE072
http://www.horizonhobby.com/Products...rodID=SAIE082A
I just ordered this one for my World Models Rambler 45 to replace a .55-AX:
http://www3.towerhobbies.com/cgi-bin...?&I=LXSZW5&P=0
I've owned magnums, OS's and Saitos. All work well but the OS's run better right out of the box in my opinion with less break-in and are a little less finicky. I also hear that some of the new Thunder Tiger 4-strokes are nice.