340 7-blade fan rivet missing

The factory did balance these fans. They did it by adding a flat washer and riveting it on.
Never saw that.

IMHO, you could buy a steel rivet and have it properly swedged on.
Sky scrapers, bridges, ships, planes, and fan blades have had rivets in use for more than a century. The process is not particularly sophisticated.

THAT BEING SAID... for the money a replacement would bring piece of mind. You might even find one with a more agresdive pitch.