4e is all about speed of combat (at least in theory), so you might want to take out some of the more fiddly 1e combat rules such as "weapon vs. armour type", "weapon speed", and so forth if you haven't already. Also for your own sanity re-build and streamline the combat-matrix tables for the various classes.
4e also does terrain, movement, and set-piece battles quite well; perhaps take a look at adding some sort of mechanical advantage to movement - maybe something as simple as movement into combat before attacking gives +1 to hit, movement out of combat after attacking gives 1 point AC bonus, etc., but you can't do both in the same round vs. the same opponent.
4e tends not to bother much with resource management, so to keep that feel you'd want to reduce or eliminate such things. Assume everyone always has a light source, for example, and enough rations to see them through. Throw encumbrance rules out the window unless someone tries something ridiculous like carrying an anchor in his backpack. And so on.
I'm not sure how well a skill challenge mechanic would work - you'd first have to add in all the skills along with a mechanism for gaining them. Probably more of a headache than it's worth.
After that, I can't think of anything that wouldn't need a design-level rebuild of at least part of the 1e system.
Lanefan