Ditch lights are not required as long as you stay under 20 mph over crossings, which allows many tourist/historical operations to ignore this requirement. It is interesting that a "historical" locomotive according to their definition is only 1948 or earlier, meaning a 60 year old locomotive does not qualify.
Since Portola often takes their enignes out on the main, they have equipped some of them with removable ditch lights that just bolt on to the pilots. This might be the best solution if there might be a future opportunity for mainline operation.