In my humble opinion, putting much money or effort into a T&S 758 isn't a good idea. It has plastic gears, so even if you get the timing belt installed, you still have a machine with plastic gears. My 778 (same as a 758, except it's a free arm) is still working and I love it, although I don't use it very often as I prefer a machine in a cabinet.
Several years ago, I bought a bunch of sewing machines at auction including 3 or 4 T&S. They all had shattered gears. Leon sent me a 756 (for my quilting kids class) that was working fine. The gears shattered within a month. They are on the last years of useful life, as they are from the early 70's and so are 50 years old or so.
So, I guess my decision to repair it would depend on the cost of the belt and how much I enjoy messing with stuff.
Also, Dublb mentioned working on a 600 and 603. They both have metal gears, so would be worth fixing.
bkay