I assume you have older (pre- DCC) Kato chassis. Parts for the newer chassis are usually available from KatoUSA (or were, last time I needed some).
Are you missing the "floorboard" (the part that clips to the chassis) or the "coupler clip"- (the part that holds the coupler itself into place), or both?
My recollection (and please double check me before purchase) is that the old, out of production, 1159 conversion kit included its own coupler mounting that replaced some of the Kato parts . This kit only works with pre-DCC kato chassis. The 1159 are so rare nowadays that I could not find a current eBay listing (although there is a lot of stuff I can't find since eBay "upgraded" its algorithm). But you might find some at one of those old hobby shops where there are boxes of old stuff to search through.
The newer model conversion, 2000-1, is just couplers and uses the Kato clip. Instructions for the 2000-1 can be found at MTL's website.
I'm really surprised that no one has replacement parts on Shapeways- the front end parts for older Kato F units are probably the single most needed parts in N scale- how many of us have at least 1 frame in need of a coupler clip or floorboard?
