Ah c'mon, she could be the only little girl on the block with spinner!
For the center shafts, some of the big Savages and LSTs have some rather long center shafts and will hook up to your Truggy diff no problem. Only problem is that they aren't telescopic so you'd have to watch your length really carefully.
You'll probably have to put in some kind of suspension, the front "axles" on that are also the supports. Maybe cut the part off that goes into the wheel but use the rest to support it. Fitting 17mm hexes to it could be difficult, you could find a 17mm socket or something, cut it to size, and weld it on but they you still have to hold the wheel on with something.
It might be kinda cool to get her one of ^^^ electric cars and build that up. You can find them at garage sales for $50 or so and they already run on 540 and she can steer it (wouldn't that be fun?
). The Medusa should mount up pretty easily and there's plenty of space for batteries, all you've gotta do is figure out what to do with the throttle/ESC. Since she can steer it herself perhaps keep that part radio controlled
Looking closer at the steering on that wagon, that's a pretty shoddy design! I remember wagons as having the whole front end as one assembly that would rotate with the handle, that would steer MUCH better. I may have a front end you can have...