Merritt Marine this is a great source for those welding rods. They used to be a big Triumph dealer, and for a lot of years, maintained a stock of hard to get parts.
That crack looks pretty bad and looks like it's gonna need a lot of prep work.
Fortunately, I never had to weld any of my Triumph's. So I don't have a lot of advice on how to do that. But there are others on this site that do.
Martin