Is it a V twin? Or an opposed twin? Big difference.
There is a legend that there was a 12hp opposed twin Briggs. 40 cid I believe.
Although I think some guy bought a sticker kit from Briggs and fooled a bunch of people. Because the same guy found a 40 HP 49 cid Briggs v twin.
If it is an opposed twin get a 33 ohv single. You will smoke the opposed twin and it is identical to a ?12 hp? smaller ohv single.