Personally, I would not use this inlay on this rifle, I would use this on a late flint and/or percussion rifle. If you really want to use an inlay I think a simple oval would be more appropriate. I enjoy making my own inlays and such and it's not very hard.
When ever I work with brass or German silver I make sure to annealed the metal and to make it fit a rounded surface I simply use two pliers and put the slightest bend in them. Being very soft they bend extremely easy.