To remove the shell, there should be a screw behind each cylinder at the front, and I believe the two screws on the rear of the coal bunker (I'm positive about the front end, not so positive about the rear since it's been about a year since I had one apart).
As far as the piston, you might be able to sand it down slightly with some fine sandpaper. Neither time I repaired his was in a very good place to do a proper repair, so I just oiled it lightly with some labelle oil and got it running for the rest of the event we were at. I recall this was a common issue discussed here on the forum, so you may be able to search for old posts about it. I don't remember what others did to get theirs back up and running again.
