When you use a puller to take the sprocket off, if there is a lip inside the pulley,
where the two meet, might work with a brass punch to separate them from behind.
Some guys here have been talking about upside down air spray cans (dusters for PCs), to super chill parts.
I'd chill before torching a pair of parts.
Make sure to clean up the surfaces with some light emery cloth before re-assembly.
They get stuck no matter what you do, most times.
If you drill and tap the washer, you could use a puller to remove this with no issue later down the road.
That's over engineering though ..... and it's probably rediculously hard to do.