Do not let a Fox fanatic see that engine or they will be all over you for it. It is about a 1949/50 or so Fox and it is probably the Fox 35 and it seems to be in decent shape. The 4 bolt head and 2 bolt back plate and tapered thrust plate are a give away to its early vintage. The heads had a habit of leaking or warping and the 2 bolt back plate also had problems. They were replaced by a conventional 6 bolt head and a 3 bolt back plate. Reinforcement webbing on the front of the case came in spurts over the following years. Shame your missing the original needle valve it might have been an Austin craft but I'm not an expert on Fox engines