I'll go, any of the lawn mower carbs will fit if you take a peice of 3/8" or 1/2" aluminum plate, bore it the size of the intake port, make it a little bigger that the hole, counter sink the two bolts that will go into the holes on the block. Take the carb of you choice (I recommend one off a 12 hp briggs walbourgh) tap two holes into the aluminun adapter you just bolted to the block and drill for the bolt holes the new carb will need. Hopefully the block will have a different pattenr than the walbourgh. Hook you throttle cable to the new carb and all is well.
Beware if the 11 hp has a syncro in it, and you tiurn it much more that 3650, something will break.