There are quite a few trike based companies around. A lot of them are right here on these forums. You have the restoration guys like MudDog's Restoration, and Vintage Motorsports. You have a few guys making plastic like Hornets Powersports, Aussie Duner and others. Then you have guys like Sprock Racing and Halls Performance doing engine work and other
parts. Harlen does repro stickers (I think he is still doing them), and Syko does tank restos and fiberglass fenders. X.System does bushings, GKing does beautiful polishing, and the list can go on (sorry if I didn't list someone in here).
You can also get some
parts through ATC Heaven.
A lot of these guys you can find threads about in the "Research and development" forum. We should have a sticky, listing all of these fine members with links to their websites.