We (Jason and I) investigated that (or a similar) ballast car that was available for sale about a year ago. In short, it would have required too many modifications (especially with the trucks/wheels) to be usable for the WW&F (or similar two-footers.)
They do come up on occasion, but are usually scaled for backyard, mining, or amusement park 24" gauge railroads. The only true solution is to build one scaled properly for the Maine two-footers (like the one the Portland Co. designed, but never built.) That means new (or rebuilt wheels, trucks, and couplers) which are high-priced and rare commodities.
In reality, we are almost done (relatively speaking) with this current sprint of track-laying. Any further (large scale) expansion should probably be predicated by construction of a suitable ballast hopper.