You've got a couple of obstacles that you'll need to overcome if you want to change those cranks. They are cottered cranks so you'll have to replace the bottom bracket if you want modern replacements. That would normally be a fairly simple matter except that your bike has a french threaded bottom bracket and you can't get a modern replacement with french threads. There are a few solutions, but unless you are worried about the weight of the cranks, I'd just clean them up and keep using them.