I think Service Honda lists gasket sets. They can also be found on eBay, but be sure to get one of the better sets that has metal shim gaskets for the head and head cover.
Originally Posted by skeptic
I am not positive, but I think you have to split the cases to replace the shifter shaft mechanism. If that is true, and the bike is otherwise not in need of repair, why not just weld the shifter to the shaft for a temporary fix.
He said the shaft itself is moving 120°, which means it broke inside the engine. And yes, the cases have to be split to replace it.