I'd like to know more about that Caswell AL aluminum brazing rod product;it seems like good quality.
I'm sure a Tig welder who often works with cast aluminum will fix that cover for you TT.
Can you check to see if the bottom surface of that rocker cover is still flat(just in case the PO warped it by forcing it off)by putting it on a machined cast iron table and checking it all around with a fine feeler gauge ?
I had crankcases Tig welded at a Technical High School in Indiana,PA. when i lived there by a shop class and the teacher prepped the parts to be done in an oven multiple times.He kept putting the parts in an oven until the cast aluminum parts stopped having oil come-out from the aluminum pores.He said this was necessary before Tig welding the parts together.The parts came out very nice and I'm sure the oven was necessary.
TT,I doubt you'll have much oil where the piece broke off as it's a fresh break,it doesn't look like anyone drove the bike around after it was broken? The engine cases I had were damaged on the base of the engine and had sat with old sludgy engine oil seeping through them for years. TT,you'll be fine to have that piece Tig welded back on.
I think your broken bolt mount on that top rocker cover will fit together exactly to match-up for a good,true fit,imo.
The only problem I had with their HS class doing the repair was they were vertically split crankcases and the repair/shop teacher was told by me "only weld UP TO the factory split on both cases,I'll work with the rest" because the flat factory machined split was where the cases fit together and those areas were in good condition(a dirt bike engine)and it had a gasket between the vertical split on both cases. They didn't follow my directions and welded all the way around the cases
