Tony Sportiello, who wrote the book for “National Pastime” at Keegan Theatre, must have had fun coming up with all the silliness that constitutes the musical. A work that breezily begins with a far-fetched comedic idea, “National Pastime” is the story of a small Iowa radio station that is going bust in 1933. Sportiello’s solution is to have the station, WZBQ, broadcast baseball games between a nonexistent home team, the Baker City Cougars, and endless European teams. The trick is that the games are always played away from home, so that through the magic of radio they can be created by two men in a studio. The public never catches on.