Your experience is typical...the toilet is over 40-years old. What's the per year difference if you spend a little more to get one that works well? A dollar or two a year? For something you use every day, often multiple times a day, is it prudent to skimp? The old one probably uses 5-gallons or so to flush, the newer ones max at 1.6, so there can be a huge savings in water and sewer costs, depending on where you live. Even if your costs are low, the environmental impact is big. Many utilities offer a rebate to update an older toilet. Look into that, and then compute how it affects your budget.
Not sure on that one specifically, but most toilets can be repaired or rebuilt. Your question, should be, is it worth it?