If you look at `:h f` you'll see 			

> |:lmap| mappings apply to {char} 

(as in `f{char}`)

Go to `:h mapmode-l`:

    ":lmap" defines a mapping that applies to:
    ...<snip>...
    - the argument of the commands that accept a text character, such as "r" and "f"

So `lmap <c-j> <space>` ... note, though, that you may need to type <kbd>Ctrl</kbd>+<kbd>^</kbd> before using any lmap mapping.