Need help with Id and replacement bracelet for Omega Constellation

The reference number of the watch will be on the inside of the caseback. You’ll need that for absolute certainty, my guess would be 168.044, cal. 1001. There will likely be multiple bracelets that fit this ref, I’d say 1202/205 (with 20mm straight endlinks, of course) would be a good bet.
Note that this likely hasn’t been serviced in ages, so if you plan on wearing it you should have a watchmaker do so first to avoid damage on the movement. The 100x movements don’t always have the best reputation regarding reliability, unless they’re maintained properly.