If you guys will post some more info, I can help. Engine model number, carb numbers and so on. What parts you need.
The reason Onan parts are all over the place is because they don't make them anymore. Onan parts are almost NLA, so your going to be subject to the seller. Kinda like Continental motors. Yeah, you can still get the parts, if you can afford them. I appreciate the fact that both you guys want to keep the "original" motor in the tractor, but there are a lot of good reasons to get away from a Onan. Namely, parts availability. If you are dead set on keeping the Onan, your going to just have to accept that the parts are going to cost an arm and a leg.... and maybe even a big toe.
I found a carb kit on E-Bay, $155. Or, looks like a new carb is available. $300. Just how it is guys. Sorry.