Jessica,
I've wondered the same. The railroad does have the State's. Maybe they could convince them to go half and half on one? I know they are almost done with the track plan which was the theoretically point to becoming self-sufficient. I know COVID-19 probably kicked the viability of that 2-3 years down the road, but maybe once the track plan is done, they could request that funding stream to purchase one. After all, in high-fire danger seasons a diesel could keep the railroad afloat and self-sufficient To keep the schedule, they'd need two of them, but just one could be a life saver.
I'm not sure the likelihood that the Forest Service would shut down the railroad. The C&TSRR has two speeders follow each train, and even have water cars that could be (or are) modified to spray down the track behind the locomotive. The railroad also is in a more remote area than the D&S. That being said, it COULD happen.
I bet once the D&S gets these two diesels up to snuff and then get the two new locomotives, they might part ways with #1203 and another one. I'm not sure if they'd get rid of "Big Al"