no problem, another option as well, but pre dates your version of the bike by a year or so, is Fox did offer the Trail bug with a 4hp medium frame Tecumseh on it. They mounted the stock steel tank off the Tec medium frame engine onto rear of the frame. I wasn't sold on that ever actually making past the brochure because they offered a 4hp briggs at the same time , but, as you now know though the briggs 4-5 hp wont fit that frame
but one did in fact show up recently either on here or facebook that still had the tank bracket on it at least. Could do a stand alone float carb on it that way in a pinch.
you can see it here bottom left of the 1967 brochure: