I have a Champion 120V generator, I want to use it to plug into a preinstalled/wired 240volt split phase outlet
Nope, no chance.
You can't get there from here.
The only way to do it involves some wild back-flips with a 5 KVA 120/240V transformer, but those are $400+.  (you can occasionally get a find on Craigslist, but even then, the price will be greater than the price diff between this one and the generator you need.)
So back it goes.... if the well is a priority.
If it's not a priority, then maybe we can hook this up, but we'll need to see a much more detailed look at your generator interlock or transfer switch equipment, and how the panel is wired.