I'll just throw this out there, my 1650 is one of the earlier models and it had the small cap/neck with gauge.
I found it a pain to fill, so I went to the larger, later tank/cap.
it's much easier to fill and I got the cap/with gauge,tank, & shutoff for $15 @ a tractor show.
They do sell the small cap on ebay, I've seen it and it fits other brands of manufactures that use Kohler engines.
Now if your doing a numbers matching concourse de elegant restoration, ya will loose a couple of points for the wrong tank/cap if you substitute.
