As with a number of Pantera issues, the answer is 'Yes, but....' Larry Stock at Pantera Parts Connection rebuilds shattered latch buckles, but your 49-year-old OEM cloth belts need an extremely close inspection for frays or any other damage before reuse. The Navy tests (or used to) aircraft seat belts on a 5-year schedule using a hydraulic ram that subjects the belts to 125% of spec. load. During this, a 3" wide belt will elongate some 4 inches before recovering. They reportedly test shoulder harnesses too, but I've never seen that done.
Cosmetically, I've had good experience by taking a stock but dirty Pantera seat belt and dropping it (buckle and all) into a sonic cleaner with some dishwasher soap & warm water. After 20 minutes or so, I let them drip-dry overnight. They came out looking new! Cross-shoulder straps should react the same. I think there are (or were) shops that rebuild belts using new cloth and rebuilt hardware, but in today's litigious climate where kids sue their parents, that's a Google question.