As I mentioned in my earlier post, I'm not sure why Hopey said what he said, but as this is a boombox forum, maintaining, restoring and repairing boomboxes is what we do here.
As for your boombox, you already have the service manual so you have the same information that any of us would be using as a guide. No, these publications almost never provide hand holding and presume that the user is a technician with good mechanical aptitude. Generally speaking, you would have to just dive in using the instructions on how to remove the cassette assembly from the boombox. At that point, you'd be well advised to snap lots of photos as breadcrumbs to put it back together again. Unless the belt is completely missing or disintegrated, the routing should be obvious. If not, you can generally figure it out. The service diagram does show how the drive belt is configured and the general shape. Due to the manner in how these exploded views of the mechanical section is, it's not really possible to show the belts being routed (very few do, they almost always show the belt floating in mid air, but they will be pointed in the general direction of the pulley they will be attached to. Sometimes, the whole belt isn't even shown, just the ends cut off but pointing to the pulley they will go to).
As for belts, I'm not sure what service manual you are looking at but my copy does have the P/N, which you can either cross reference online via sites like turntableneedles, or you can simply measure by looping a thread through the belt path, spead it out and measure it, and deduct about 10-15%, which should get you right into the ballpark. The pulley size and shape generally tells you if it's going to be a small, medium or flat belt.
Lots of boombox hobbyists have experimented like this and sometimes, it may take several tries to get it correct. Just work at it, persistence and elbow grease, and you should be able to get it working again.