I believe that Scarebird had a conversion kit for the 9 inch drums to mount disk brakes on them. But, I don't know if they are making them still.

As mentioned above, the better bet is to convert it to the 73-76 disk set up. The 9 inch set up really isn't very heavy duty at all. The only caveat is that you'll also have to convert from the small bolt battern wheels to large bolt pattern wheels.