A 12x8 at 13k rpm is about what a .90 size 2 stroke glow would give you, so that setup is way overpowered for your plane. At 10k that's about a .90 4 stroke, which is still overpowered. I'm guessing you're using an 11x5 on your .46, probably turning about 14k in the air? If you choose a power system that turns that prop (electric version of course) at that speed you'll replicate the performance of your .46. If that lets you keep the weight the same, you're good. If it gets a little heavier, you'll need to consider how happy you are with the power the plane currently has and consider is a little less performance is ok, or it you will want to go up a few Kv and draw a few more amps to compensate.