ENYAs are very good engines and last a life time.
Try to soften the old castor oil putting the engine in the oven for a few minutes at 180 degrees.
If you can remove the carb, you can stick only the carb in the oven, if the engine is free. In this way you can remove the O-ring (if any) between the carb and the crankcase and save it when time will come for reassembly.
Beppe