After 12 years in retirement I have got around to cleaning up my model motors that have been sitting in a box for the last 15 to 20 years. Castor oil, old, burnt or varnished up is hard to remove.
So I used a caustic cleaner to soften it then boiled the pieces (where it was OK) to further loosen the gunk. Boiling was also needed to unstick the pistons and crankshafts. A brass wire brush was used to remove the worst of the rubbish and then I dunked the parts into kerosene to get rid of the water.
Finally I had to use a knife or a brass wire brush in the Dremel for stubborn deposits and then finish up with a light steel wool (fine grade).