To some degree it depends on what languages you are fluent in other then english. Presumably you speak japanese, but I don't so I have no idea what may be available there. The best funded engine research these days is done in F1, in Germany and Italy (other then maybe Japan and now Renault in France)
They don't tell you everything, but being one generation behind these guys will beat most others. You just have to find what is applicable to the world of Harley and what isn't.
It also depends at what level you want the info. You can get anything from Ricardo to the V-twin tuners handbooks, but written info, unless it is basic research, is obsolete by the time it is eventually published.
The most up to date written source in english are SAE papers.
THIS is one of the sharper guys in the US.
These days almost ALL technical information is equally available to all, it just comes down to differerences in philosophy, money and what you do with it.
You can find most anything you want to research with a good search engine like
THIS