It is probably a 1980 KX250 Unitrak engine. The number is consistent with the A series of engines which ran from 1978 to 1982. There were about 1500 1978 models made, and about 2500 1979 models. Therefore it is right on the border between 1979 and 1980. It is easy to tell the difference - the 1980 model has the clutch actuator behind the cylinder on top of the cases, the 1978 and 1979 models have the clutch actuator on the clutch side of the engine near the front of the clutch cover. Also the 1980 has a single large mounting lug at the rear of the cases where the swingarm bolt went through, the 1979 used a cradle which linked to the swingarm. If you post a few photos I will be able to tell you straight away as the motors look quite different. Cheers.