Before you do anything, find out about your rifle. For all you know, you may have a very unusual, special order one-of-a-kind rifle worth thousands IN IT'S ORIGINAL CONDITION. Get the model and serial number, and send it in to the 'Buffalo Bill Historical Center' -
www.bbhc.org. They can tell you if it's a run of the mill rifle or something special.
If it's a boring old rifle, spend $30 or so and get a proper set of gunsmith screwdrivers so you don't murder the screw slots. Disassemble it. Soak the metal in BreakFree CLP, or Ed's Red, or a synthetic gun oil, but NOT WD-40. CLP especially will literally dissolve the rust without any mechanical action. For the bad spots, gently use 0000 steel wool and lots of CLP. Use toothbrushes, toothpicks, hard plastic, brass brushes, that are all softer than the steel you're cleaning. You can also use spray brake cleaner to flush junk out. Finish up with a light coat of oil.
For the wood, use simple wood refinishing techniques like fine steel wool and strippers. Get the crack fixed first, though. If it's a non structural area, simply glue may work fine. Otherwise you may need to go to a gunsmith for a more solid repair. DON'T just put a couple of deck screws into it. Don't worry about the dings and dents, they are beauty marks. Sand it with 220, and then choose your finish. Boiled Linseed Oil will transform the wood into something beautiful, as a suggestion. Tung oil doesn't need stain either, and really brings out the grain.
The rifle bore is another challenge. Like most old-time rifles, it probably wasn't shot very much and so shouldn't have much copper or lead buildup. Take your time with the usual cleaning routine of brushes, different solvents, patches, repeat.
A rusty old abused rifle is a sad sight. Even after it's all cleaned up, it will still show rust pits, dents, scratches, dings, cracks, and gouges. But the metal will be shiny and clean, the wood will come alive again, and suddenly you can be proud to display (or better yet, SHOOT!) a wonderful piece of history.
