I don't think that the OP's crank has a replaceable large chainwheel. As others have pointed out, the problem sounds like it's more likely caused by a worn chain, and maybe freewheel, than the crankset. Though maybe the bke shop, if they diagnosed it, rather than simply responding to a query about what a new crankset/BB would cost, knows more than the collective gray cells here.

If the crankset is, in fact the problem, then I would suggest buying some vintage of Shimano 600 crank on eBay, and an IRD cartridge BB, for which threaded inserts are available in any threading (English/French/Swiss) the BB might be.