My blower box was replaced about 4 years ago after sucking up a end cut from a oak tree. The tree service didn't pick up all the stub cuts...........cost me 400.00 to replace.
The manual isn't always helpful as words do not tell you of troubles. Rusted bolts, pullies, and pulley pins can break or shear. I would let your repair shop do this task unless you have a full day to try your luck.
You haven't tried the starter yet.............you haven't tried to rebuild the deck gearbox have you?? The walker is a unique mower that must be taken care of. It is tough and it is tough to work on. Most if not all major replacements on this mower can and will take a single person 3 to 4 hours to perform.