"Then we follow the Pony Express Trail heading east and north. I actually have no idea how much of the Pony Express dirt trail we can follow and if any of it is closed to motorized vehicles."
You can ride all of it, the only place that will probably be difficult on your air heads will be Overland Pass (Diamond Valley to Newark Valley). If you have trouble getting over it, just by-pass it via Railroad Pass and link back up with the trail.
There were still some snow drifts visible on th north faces in the Toiyabes last week, but it has been pretty warm the past couple days and getting hotter, so Ophir may be doable.
Have fun, sounds like an adventure in the making....