I have a 5500 and a 500 that have the left hand worm drive. Both those saws also have the 4 screw recoil cover also. I don’t know how reliable an indicator this is. I have a third saw, a 500, with the late recoil but have not pulled it apart to check the worm.
A std M20 nut is only 16mm thick. You need to reduce the diameter to around 25mm for 12mm or so to fit down in the pump recess which only leaves 4mm of hex left to grip the nut. A deeper nut or welding a tube on would be a definite advantage. With a bit of planning you should be able to use the same tool to re-install the worm too.
My initial plan was to use a small puller or bearing separator to grip the nut, but instead I used an M24 x 1mm washer over the recess to spread the load, locked the crank, greased the threads, and cranked the worm off with a spanner. I did this twice without issue.