Depending on the components, you likely don’t have to replace the whole crank… just the chainrings. Is the bottom bracket (bearings where the cranks go through the frame) in good shape? Has it been serviced regularly or only when it makes really bad clunking noises? Cogs of that vintage are likely cogs, and not a freewheel. My guess is that they’re 8-speed. 8 speed cogs are still available, though not normally in the higher end components. Modern cogs could be used with 8-speed spacers, but it takes a slightly creative mechanic to make that happen.
Hope that helps.
Pete
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