Help needed with starter issues: 1998 argo response 8x8 18hp b&s engine

[rentalrider]

Guys, I need help. I have a '98 response with the 18hp b&s. Was turning over and starting fine. My son drove around our property in the deep, fresh snow (real wet, heavy) for 1/2hr or so. He parked it without issue. I got out of plow truck and jumped in with him and when I turned the key it just spun. Never engaged engine/flywheel. I went up front, bumped with a screwdriver and no change, just spinning. I tracked a wire from starter area that went to front of tub which had a 30amp fuse which seemed to be melted. I removed the fuse best I could and tried again, same exact thing. I've replaced this fuse section with new and put in a fresh fuse. No change. I felt under the front of motor and can feel a rectangular piece which seems to spin on shaft of starter freely. It however doesn't seem to engage (if it's supposed to). I tested the starter solenoid and it gets current across the main terminals? Didn't seem right, but I'm not sure. So... machine runs great when started with recoil but just spins freely when starter is triggered. Help??? Thanks in advance...