If I had to guess, I doubt it would be a round imperial figure like 1/4". I don't know specifically what size Honda used, and it was the mid-60s when some standards were changing, but the 5.5 mm fuel line that is standard on many Hondas not long after that should fit it properly. It's been my experience that 1/4" always requires a clamp to seal, which complicates things as the fuel line gets older.