One thing possibly worth looking into is that bike fork steerers for quill stems only come in a couple of diameters and threadings; aside from head tube length, they're fairly interchangeable. You can probably track down a much beefier fork that will still work fine with your stem and bars.
Also, if you've got a source for used aluminum crutches, a Google search for "bicycle trailer aluminum crutch" should give you lots of ideas for getting the load off the bike altogether. If you're riding rough areas on a regular basis, I'd highly recommend a single wheel design.