Most probably an engine that used a castor oil based fuel. Just heat the engine up and it will most probably free right up. You can use a heat gun or better to just put it in an oven and heat it to about 250 degrees. Use some gloves and it will free right up if it was because of castor oil. Once free, it will be wise to check it out more closely by either partial or complete disassembly to check bearings etc. Once free, a liberal application of ATF (automatic transmission fluid) or other good oil will lubricate it and keep it free.