As Jim noted, it is a US-cased model, so the OP will probably not be able to use a reference number to order the crystal. Personally, I would just use a generic acrylic crystal for a watch like this. A professional watchmaker will have an assortment, and can fit one for you.
If you want to be certain that the strap is made by Omega, you may be able to get a strap from an authorized Omega dealer. Personally, I wouldn't bother, it means nothing to me, and again it doesn't make sense to pay the price for an Omega-branded strap for a watch like this. I tend to avoid vintage straps, since the leather has sometimes degraded.
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