That's a really great projectile! You never see them with such good lettering. Its too bad some idiot screwed it up by removing the band. There are so many people out there that destroy projectiles that are worth a lot for a dollar's worth of copper.
I'm not sure if you will be able to get the band back on and get it to close tightly around the projectile. These bands are normally made up as a ring that slides over the projectile, and then are hammered into the groove or swaged into place, and then turned on a lathe to the final dimensions. The piece you have is going to have some spring in it, so it will want to stay partially open, unless you can solder the gap together, and I would be worried that the heat needed to do the soldering would ruin your nice paint job.
It is also going to take a lot of force to bend the band. Even though copper is soft, the band is thick enough to take a lot of force to bend.
You might get it to fit easier by cutting the band into two equal pieces, then close the two halves so they are the proper radius to fit the groove just right, and then put them in place. You could epoxy the two halves in place, and then touch up the gap with some liquid metal epoxy etc. Just a thought to save a lot of frustrating effort.