I thought about one of those Barbie Jeeps, but I already have the wagon and it's rather unique to R/C-ize a wagon.
Anyway, after doing some more research, I've settled on using the Summit as the base of this project. The transmission allows LOTs of gearing options and includes a motor mount. The wheelbase is close to 18.5", which is really close to the wheelbase of the wagon I'm using. And being a Traxxas product, replacement parts are available everywhere.
So, does anyone have just the front-end and transmission assemblies of a Summit they want to get rid of?

I could get the whole thing, but I'd be parting out and selling about 70% of it, and it's rather expensive...