Kroil.

Using Kroil has never failed me, and I have had things stuck so bad you run out of curses.

Here's what I would do if I had your fork.

1. Wash it out real good in hot soapy water. Completely degrease it.

2. Warm in oven or use hairdryer to fully dry it out.

3. Douse it real good with Kroil.

4. Let sit for a day and douse it again.

5. After day two, tap the stem in both directions using a brass punch or a brass rod. Be sure to back up what you are hitting, i.e. place the fork crown on something very solid, like a vise or a solid piece of steel. Use a brass hammer if you can, but the heaviest steel hammer will do if you are using a brass rod.

This method employs shock force to loosen and free the galvanic corrosion between your fork tube and the stem.

6. It may not budge the first day but keep trying and be patient.