Well, now I have my 540 and have taken the bobbin 'area' components apart, with these conclusions:
It looks as if the 540 rotary needle and bobbincase are the same and will fit the 117. That's good! (The bad news is the cost of the ones on ebay!)
Take a dekko at this pic I've just taken of the 540 components.
Sorry if the pic isn't showing... it does in my preview, -I'll see if I can fix it.
I don't have the retainer circled, it holds the edge of the round backing plate that is screwed to the back of the rotary needle.
I'd need one of these retainers, though I could make one using this as a pattern.
Note that spring 'clip' thing arrowed. I can't see any function for this. It's screwed onto the 'box' and extends around to the side/rear of the rotary needle. The end of the spring spiral is not broken, (it is shaped and 'finished' but doesn't seem to go anywhere!). It wouldn't fit on the 117 anyway because the 'box' screws are arranged slightly differently.
(Sorry, I'm calling the 'box' the part of the underside casting with the square-ish cover plate, through which the shaft passes for the Rotary needle)