I normally don't worry much about rust on the crankshaft all that much, unless it interferes with getting a bearing on or off.
I use some strips of  super fine 800 grit emory paper on the crank to get any crud or rust off that stops a bearing from going on OK Just enough sanding  to allow the bearing to go on.  But one can use Naval Jelly to good effect.  Using a wire brush is useful too, especially if you get some wire brushes for your dremel tool.   For old castor oil that has aged and congealed, some Dawn Power Dissolver works well. I spray on some and let it sit for a few minutes and use a tooth brush to scrub it with. Reapply as needed. Now removing the rust still leaves the pits though. But in any case I only remove enough crud to let me replace the bearings, etc. I see no reason to go all anal and get it to look like new. The rust will prevent that new look anyway. Plus the old castor prevents rust, so leaving a thin layer on the crankshaft doesn't hurt anything. Most of the time, instead of rust, you have old castor oil layers that have built up over time and congealed, so there isn't rust per se on the crankshaft.