No the plates dont come off...its pretty much a 1 peice sealed unit it seems.
Not sure about starter solenoids...there's gotta be a direct replacement for these out there somewhere...OR someone has an old Champion collecting dust they want to part out?...
I found a similar looking one at AutoValue, it has the 2 big and 1 small wire terminals like these, but it mounts on its side so they would have to sit loose inside the winch..