I took the left valve cover off and did find a broken plastic piece. This is unfamiliar territory for me, so I've attached some pictures and am hoping to get some thoughts as to what is being shown. If the pictures don't come through or are too small, here are links to the full-size images.
www.tobogganhill.com/420sel/420_rail.jpg
www.tobogganhill.com/420sel/420_rail2.jpg
www.tobogganhill.com/420sel/420_rail3.jpg
www.tobogganhill.com/420sel/420_plastic_piece.jpg
I look forward to opinions about whether this is worth further investigation or if it is clear that valve damage (or worse) is likely.
The timing chain is loose, but didn't seem horrible (maybe 3/8 to 1/2 inch of play - or maybe that is horrible :-).
-Brett