AXi provides application charts for all their motors at
www.modelmotors.cz. This information helps immensely with figuring out what prop/cell combination works without having to try a whole bunch of different combinations. Plus, the numbers are fairly accurate. It's one of the two main reasons I'm a big fan of AXi motors.
We've really been going about this whole conversion all wrong. Henke recommended a series of motors, left you hanging, and we've been concentrating on discussing motors ever since. You can't pick a motor until you know how much power you need. With electrics, you can dictate exactly how much power the plane has, and you must do that, or you're just poking in the dark. Electric power is measured in Watts, which is the metric equivalent of Horsepower. Electric Watts translates directly into mechanical Watts to spin the propeller, through the motor.
What's this plane weigh? Maybe two pounds at most? For normal glow sport plane conversions, I generally recommend 100 Watts per pound to get comparable glow sport plane performance, but this is an underpowered floater. I have a feeling that 50 Watts per pound will make it seem like a hotrod. Two pounds, 50 Watts per pound, 100 Watts total. 100 Watts spinning as large a propeller as possible will haul this plane around like a rag doll.
Let's look at the AXi 2814 series motors, and their suitability in a 100 Watt application. We'll start with the 2814/10. The lowest-power combination is 7 NiCd/NiMH cells (2S LiPoly) and an 8.5x6 prop. Current draw is 22.5 Amps for a power of 138 Watts. That ought to be plenty.
AXi's application charts allowed us to kind of skip a step, which is to manually size the battery, but that's okay. We've now got all the information we need. You'll need a 7 cell NiCd or NiMH, or a 2S LiPoly that can handle 22.5 Amps, a brushless ESC that can handle at least 22.5 Amps, the AXi 2814/10 motor, and an 8.5x6 prop (or something in that size range, from 8x6 to 9x5).