At the same time, don't be fooled by displacement numbers alone --

The 40fp is optimized for reliability, lots of focus on smooth idle rather than on max power. My recollection is that it was rated at about 1.1hp. A 46fx is focussed much more on power and is rated at about 1.6hp -- so the .06 cu.in. increase in displacement doesn't sound like much, but combined with the different design approach, you get 45% more horsepower out of it.

Note -- don't mistake this as meaning that I think you ought to buy the .46... it's neat that it has so much more power, but the more important issue is still whaether or not you need more power.