Why not one of the .40-.46 replacement motors? That would give you equivalent power and shouldn't make it too heavy. The Rimfire .46 would do fine and wouldn't be stressed with any reasonable prop you might pick. The Himax HC42 is another choice. It should do nicely with a 4 cell pack, probably a 4000-4500 mah and a 12x6 prop at just over 12k rpm when fully charged.