I figured out how to make this work in 2.1:
First of all, there's the loading bits, starting at line 212. Comment out everything from
// Just some boring admin stuff
// This is the most important bit of the whole plug-in. This actually is the marquee!
Just below the "boring" line, put this:
add_options_page('WP-Marquee Options', 'WP-Marquee', 9, basename(__FILE__),'wpmarqueewp_options_page');
On line 82 there's this bit:
Most Current Stable Version = followed by some "required" business. Whack that out (all the way to the end of line 82).
It also bugged me that there are single quotes in the body of the plugin options code which confused my code editor. I changed them to
" as appropriate.
It's not perfect, but it does work. Mostly.