Freewheels are pretty easy to put back together, in the grand scheme of things. The trick is that most people now don't know how to work on them; there is an excellent article written about it on Sheldon Brown's website, but the gist is this: the most difficult part is re-seating the pawls without dislodging any of the bottom (inner) row of bearings. You can trawl ebay hoping to find a pawl clip to hold them down while inserting the freewheel shell, or you can use a thin strip of inner tube with a piece of thread tied around it. Put everything together, pull out the tube with the thread, and load the top bearings. You'll take care to note that the rows of bearings are usually full compliment, minus one or two--this is normal.
What everyone is trying to tell you, though, is that it is almost never worth doing for anything but a rare vintage freewheel, considering that by the time they are making strange crunchy noises, they're usually beyond fixing by cleaning.