Unless I hated the way the car looked (because of the rust you mention), and
based on your description of the car's condition, I personally would gamble
on the car lasting at least another 50k miles and so install a new timing
belt for the few hundred dollars it costs to do so. Even if you pay as high
as $600 at the dealer's for a new timing belt, that's still only 1.2 cents
per mile, which roughly suggests it's a good investment, car-wise.

Elle
91 Civic owner; 165k miles; planning on keeping it about five more years
(maybe longer); will install a new timing belt by myself in 2007.