Im looking for a good used carb for a Kohler Magnum 20Hp (MV20S)

As long as its a inch an a bit Walbro WHG carb with the high and low speed needles, it will work.
So any opposed twin Kohler horizontal or vertical.

Mine is cactus, beyond a kit.
The high speed needle has rusted and it is not conical anymore, so it will not be tuneable. Im sure the seat is just as corroded.

Or at a pinch, if you have a high speed needle to suit, I might chance trying a rebuild.

You cant see the needle in question as its just under the float.

[Linked Image]

Not exactly the same but from a page I found on the net
Outdoor Power info - Kohler Walbro WHG
[Linked Image from outdoorpowerinfo.com]
[Linked Image from outdoorpowerinfo.com]