When I made mine, I lengthened the fuselage sides (cut new ones) by about an inch to keep the prop in the original location even though my motor was a lot shorter than an engine. This step isn't critical, and if I were doing it again I might do it the same, but I might just use standoffs to position everything - it allows for a lot more adjustment later on.
The big change you want to make is adding a battery hatch. I did not, and I have to remove the wing to change packs. If I build another, I will make the entire top section of the fuselage forward of the wing removable. Use a popsicle stick or dowel to pin it at the firewall, and use a magnet or canopy latch to retain it at the front former. Then you'll have great access for doing wiring and changing batteries.
As for the motor, mine uses
this motor. Different color, but same specs. It works pretty well and I've GPS-clocked the thing at 75 MPH, but it's a bit light on thrust for my tastes (maybe 30 ounces; the plane weighs 31). I'd probably go for something a bit bigger, or something the same size on higher voltage. I haven't kept track of the rapidly-changing supply of Chinese motors for the last year or so, so if you're on a tight budget someone else will have to chime in. My next Wonder will probably have something like
this Cobra running on 4s packs with an APC 7x6e, or if I wanted to save an extra ounce and push the motor to its limits, maybe
this one on the same battery and prop.
An interesting note on the prop: I've compared dozens of power systems for Wonders and every single time, I come back to the APC 7x6e prop. My current Wonder uses that prop, both of the systems I just listed for a future Wonder use that prop, and every other option I come up with uses that prop. Voltage, current, and Kv all change, and somehow I always end up back at that same prop. Unfortunately, it's really hard to find in stores (my local shop insists that size doesn't even
exist). You'll have to buy them
direct from APC, which fortunately isn't much hassle.