king arms 1928 thompson hop unit?

[remus]

does anyone know what unit i need to replace the hop unit in my king arms 1928 chiago typewriter Thompson as i noticed on of the screwed in arms is cracked and id like to replace/ upgrade it before i do anything else with it? or where i can get one...

edit, just realised i posted this in the wrong place if on e of the mods cares to move it to the right place please?