Rough idle turned to crank then ka-thunk

Well I'm gonna come around from the other side.
What if the engine flooded to a stall, and one cylinder happened to have a lot of fuel in it. That would cause the crank to go "thunk" trying to compress the fuel.
But as the engine sat, the fuel drained into the oilpan, and then it cranked fine.
Now, having washed all the oil off the cylinder walls, the tired old engine can't make enough compression to fire up on.
This explains all your symptoms including the original rough running.
I mean; what if.

If that is what happened;
the cure could be as easy as to fix the carb, oil up the cylinders, and probably to drain the oil.