In retrospect, I guess I should have gone with that V8, but I wasn't planning on needing a new engine...
In reality, the cheapest/fastest thing would be to sell the bloody thing and move on to another project that is not rapidly resembling a black hole. And that's still on the table.
However, an awesome member of the forums has stepped up and offered to help me go get this now-oversized-lawnmower, so at least it will be in my general vicinity so I can unload all the boxes in its back and figure out, in person, what I want to do with it. Coincidentally, this gent also has a few take-off petrols and a 200tdi laying about.