This may sound strange but I had a fellow bring me a Kubota zero turn mower with Briggs vertical twin that was only running on one cylinder as evidenced by using a laser thermometer on the exhaust Pipe. Working cylinder would hit 400 degrees in short order while non-working would hit barely 200...
Not sure if this will help you or not but I have a 19.5 hp B& S boxer, looks like yours with cast aluminum intake manifold.
Model 42E707 Type 2275 E1 Code 0106115A
The engine # etc. on mine is attached with a riveted tag to the top engine cover. Good Luck.
Are you using fresh fuel? I tried my chainsaw this past fall, fuel & spark—no go. I then took some from tank and lit it with a torch, it was like a candle burning—real slow. New mix and a couple of pulls, good to go!