I guess I would try teflon tape, but I don't know if sealing the threads will stop a leak around the bold head.
Another product to consider is Seal-All --- it's suitable for gasoline. available at the home depot.
When installing the new plastic washers, don't over-tighten the bolts.
Are you sure the leak is at the bolts and not the petcock body gasket?
1981 KZ650-CSR. Competition, Sports & Road.