Hi Adam,

The best place to start is with the weight of the plane (ready to fly glow) and the type of performance you expect from it like trainer, aerobatic, or 3D.

In general, a .40-size glow engine can be replaced with an [link=http://www.hobby-lobby.com/brushless-axi4120.htm]AXI 4120[/link] motor.

Specifications for Super Frontier 40:
Wing Span: 64 in. / 1625 mm
Wing Area: 704 sq. in. / 45 sq. dm
Flying Weight: 6 lb. / 2700 g
Fuselage: 51 in. / 1295 mm
Radio: 4 channels 4 servos
Engine: 2 stroke .40 cu. in.

Since this is a trainer type plane, we can use a rule of thumb around 65 to 75 watts per pound.

65 * 6lb = 390 watts which fits nicely within the operating range of the AXI 4120/14 motor. I would use a 14x8 prop on either 14-cells of NiCD/NiMH or a 4-cell Lithium pack.