The only one I know is the 350-4. The arm is somewhat different but the knuckle is identical, the assembly will fit and work on a 400 (I found some pics).
I have dug through several big bins of kickstarters at wreckers and looked at hundreds of them, and I have not found one that fits. Some look like they should work but will not fit on the shaft spline. Most have knuckles that will obviously not fit in the cramped space even if the spline matches.
My knuckle cracked open, I had it welded back together with a reinforcing washer welded on the open end (where the rubber plug goes). Works OK but looks a bit naff so I've been searching. The knuckle is a 350-F part number, only it and the 400F use it. Unobtanium.