I can make a wild guess that it originally had a large one-piece flo-jet that bolted directly to the cylinder, and he had a problem with it (usually it is flooding with the large one-piece, and you have to replace the float chamber seat, which is only easy if you read the manual first), couldn't fix it, found a junked engine with a medium two-piece, and decided it was a sign from heaven that he should make up an intake pipe to let him use what had come to hand. I'm a great user of junk-box parts to fix things myself, but I don't redesign engines to use up my junk, because I've learned the hard way that it ends up causing lots of work and lousy results.
Large one-piece flo-jets would be the most common carburetor ever fitted to ride-on mowers, and they are on many other older Briggs engines in the larger-capacity categories. If you decide that is what it had originally, I think you can get one very easily. I'm not quite so sure about an air cleaner for it, but if you get the carburetor from a dead ride-on, the air cleaner will probably still be attached. Again, if that is what was used originally, setting up the governor may just be a matter of taking the linkage parts off a dead engine.
