I love to do conversions! One of my possible future projects is the Great Planes Easy Sport .40, a similar plane to the Midstar. If I can ever find an unbuilt kit (discontinued) it's mine.

Anyway, you should start with the battery. This is a .40-size plane, so we're talking about Sub-C cells, like the ones used to make up car batteries. The best bang-and-duration-for-the-buck right now is GP3300 (brand and model) NiMH. Number of cells is what determines how much power you'll have. More cells is more power.

You need a cell count, and a target current. I like to use 40 Amps as my target current because IMHO, it gives a good mix of duration and power while keeping the battery weight reasonable. For sedate flying, 10 cells. To get performance equal to a good ball-bearing .46, 16 cells is the way to go. I would choose 12 or 14 for this plane.

For motor, the simplest way to go is with an outrunner. No gearbox takes a lot of complexity out of the equation. I like the looks of the AXi 2820/12 on 12 or 14 cells, with a 12x8 or 11x7 prop, respectively. http://www.modelmotors.cz lists a bunch of prop combinations that they've used. The Jeti 45/3 is also an option.