G'day Jim
The Bultaco 250 kart engines in 1974 were externally the same as the "square" barrel Pursang moto-x engines, but fitted with a 5-speed close ratio gearbox with ratios basically the same as the early model Astros. all the 250's were model #55 but the engine numbers started at M-501018 (for the earlier "round" barrels, not sure of what number the "square" barrel engines started but they were all M-5). Try Bultaco Parts in Victoria for a copy of the parts book. Not sure of power output but standard would be around 38hp at ~9500 rpm, same as the Pursang (higher for the Astro).
Hope this helps.
John
