Hello J.,
Mix up some Acetone and transmission fluid (50/50) and wet it down lightly for a couple of days. Keep the mixture in a jar with a tight lid if you can.
Then try loosening the square head bolts, first, carefully. Once you can get them removed, put a few drops of the mixture in the holes.
WD-40 or PB Blaster are also good, but they take much longer in my opinion.
Old, rusted parts take time and patience to get loose without destroying anything along the way.
Once you think it has sat long enough soaking (A few days is advised), use some wood wedges driven under the pulley and a hammer with a center punch to hit the center of the shaft a few times. Keep snugging up the wedges as you go. Eventually it will break loose but soak it as you go. This pulley was a snug machine fit to the shaft when new, so don't expect it to just slide right off.
I do not advise using a 2 or 3-jaw puller because it can easily crack/break the pulley edges. Cast iron pulleys like this do not bend, they crack and break.
Once you carefully split the case, the input shaft needle roller bearing likely need to be replaced. Pay special attention to all of the parts and directions they fit for the input shaft and needle roller bearings. Some bearings have a slight bevel on one end and not the other.
If there is a lot of debris to clean out of the case, try to keep it to a minimum to prevent parts confusion. (Your phone camera is very useful as well).
Star Tech can likely help with further details once you are into it. It's been years since I messed with one of these.
