I started to work on a 758, or a T&S very similar, which needed a new lower gear because some of the non-metal teeth disappeared. When, in the video I watched, the person broke out the dremel with the cutting wheel to remove "excess" metal on parts to allow other parts to be removed to get to the gear, I said, "Forget it."
I ended up taking the machine apart completely, for scrap, and I'll never touch (nor sew) another one again.
I think your problem will be at the top end and freeing up space to get a new belt over the top gear, but you could run into the same problem I encountered at the lower end.
I think if you can get the new belt on, setting the timing is actually the easy part.