On the engine question, can you measure the PTO shaft diameter of the green engine? With that info, we'll be able to tell if a suitable clutch half is available.
All else being equal, I'd fit the green engine if possible; it's an Industrial/Commercial model, with a cast iron cylinder liner, and a dual-element air cleaner. These are much more durable than the 'kool-bore' all-alloy blocks.
Hard to start - the first items to check on a Magnetron Briggs, after fitting a new spark plug, are the valve clearances, and the choke operation.
