How long have you been soaking it? Weeks may be needed someitmes. Blaster is good, but you should consider other sprays. AeroKroil is another good one. There are other good choices, plus several poor choices (WD-XX). Fill the holes (as many as you can at one time), maybe try heat.
Here is a link to a thread that starts with a stuck PTO, but the screws came out of this one:
Have owned a 106 for 15+ years. Began on a 12HP upgrade today. It is a working tractor as it is. Found a 12 HP core engine to work with. Pulled the 12 apart far enough to find out it’s a rebuildable core. Have already purchased a creeper drive upgrade from CC specialty. Along with PTO & drive...
www.ihcubcadet.com
Try searching PTO removal, and other variations at top right. THere have been a few threads just this year on this.
When I have a stuck Allen, I ping it in the center with a small enough punch to not damage any of the flats or corners. Insert the punch, and tap it a few times to start vibrations. Do not use an Allen with any wear, or a ball-end. The fit must be perfect with as much surface contact as [possible when trying to remove stuck set screws. Don't forget there should be two screws per hole. Check for more...