Can you get a heat gun? Maybe if you heat the inside of the fixed sheave, it will expand the aluminum enough to get it free of the crank taper. I'd heat it and try prying with a piece of wood. It's unlikely you're going to hurt anything, and it just might be enough to get it loose.
If the extractor thread really is hosed, the clutch will still bolt on and work fine. As you know, the problem with the SS/SRV/XLV chassis is the stupid way they have the PTO side crank run through the frame. If you ever needed to remove your engine again, you'd be right back to square 1.
You can get parts for Comet clutches on Ebay, usually for reasonable prices. You can also find them on craigslist most of the time.
If the sled were mine, I'd keep trying. Those are nice sleds and worth saving if it can be done.
LMK how you do removing the clutch.
-Strato-