That looks pretty good, I'd love to see the source code if you do manage to find it. Still though, it's the slopes and loops etc... which is going to be the ballache - those who do know how to do it tend to keep it a secret... it'd be nice to have it out in the open, sure there would be even more terrible flash games of Sonic released but it'd give some talented people a chance to show off their ideas if the engine was done for them.
If you're going for a Physics engine based solution (or at least, polygon based with linecasts), you might find this useful. It's the Actor class (fairly undocumented and dirty, sorry, had no time to revivit and clean it up) from which derives my Player class (Sonic). It contains all the collision detection & response routines.
