RE: Welcome to Club SAITO !
I can't speak to a Saito 50 but I've run a 12x6 and a 13x5 on a Saito 56. If you don't get any other replies, that could be a good starting point for your .50.
The Saito 56 and 13x5 haul my 7-1/2 lb J-3 Cub around in a scale-like fashion.
Bob