If it is 1971, it may be French threaded. Do you know the make of the crank? Then you need to determine if the taper is ISO or JIS.

I can't tell from the photo if the chain rings are worn, but given the age of the bike-that is likely.

If the bottom bracket is OK, then you would only need to replace the crank arms and chain rings. It appears that the drive side crank arm is swaged to the large ring, so they cannot be separated. If you live in a large metro area a bike coop may be you best bet. Make sure that your problem is not the chain before dumping time and $$ into this.