Just about any mag rebuilder can do it. I happen to like Spud Miller at Fuel Injection Enterprises. Consider upgrading to rare earth magnets so you won't have to recharge it. Also if it doesn't have an advance plate consider adding one. They come in 7 and 12 degrees (mag degrees).
Rocco was the US distributor of Vertex magnets from sometime in the 50's to sometime in the 70's. Their ads were not nearly as flashy as the TV stuff. But I remember them both well!
Ron Popeil was the guy that sold the stuff on TV ("Pocket Fisherman", "Vegomatic", "Mr. Microphone",etc.). He went bankrupt at least twice. this the Popeil and Ronco brands. His record has since been surpassed.
Don Zig magnetos was always the go to for repairs back when they were the hot setup for racing, looks like they are still around too, comes up on a google search.