The tender in the above photo is back with the locomotive (D&RGW 583/683) that is in the photo. It was donated to CRRM by Charles Bradshaw several years ago. Both the locomotive and tender are at the Colorado Railroad Museum. That locomotive is the only existing D&RGW standard gauge locomotive.
The tender that was with #583 is sitting by itself behind the CRRM roundhouse. Perhaps your idea may work for that tender - with the addition / change-out of standard gauge wheel sets for narrow gauge.