The above is a completely different system. What you have is the Hy-Power booster system which was made by Delco. Part # 504837C91 is for the booster. Part # 18006974 is for the motor. I can't find an entire assembly number but those two should help you get what you need.
That motor is only used as back up to the hydroboost so I'm not sure how you figured out that it wasn't working, but according to the manual, the emergency pump will only put out 800 psi brake line pressure vs 2000 psi brake line pressure when system is operating normally.
Now for your park brake. There is a lever on your brake assembly that will be on the bottom half of the assembly. I circled it in your picture here.
That lever operates the s cam inside the brake which expands the shoes. At the other end of the lever, which is out of view of the picture,there should be a clevis on the threaded end of your parking brake cable. That clevis is where you adjust it. Take a picture on the other side if you can and one of the apply handle inside the bus because you might actually adjust it from the inside.