As I am sure you have learned from experience, snaking a line with a drum trap is impossible, and cleaning a drum trap is a chore reserved only for a first year apprentice training exercise. For those reasons drum traps are no longer used so you may have a booger of a time finding a cap. The best solution would be to cut it out and install a PVC trap that wont corrode.
If you have metal pipes you could install the PVC trap and connect it to the metal pipes with a rubber FERNCO coupling.
The whole project would cost less than $20 and take under an hour to complete.