There are turning blanks made for making pens and other doodads that are made from half-rotted wood ("spalted" Maple, for example) that use plastic resin forced into the fibers under pressure. The structure of this stuff is basically the plastic.
Thin Cyanoacrylate (think super-glue) glue works on a smaller scale to consolidate small ares that might be rotted or otherwise weakened. Let it soak into the wood, and give it a spray of accelerator.
Small cracks can be repaired sometimes with this method if it can be clamped closed. If it can't be closed, a slurry of sanding dust from the same or similar wood can be mixed up with some of the glue and pressed into the crack to fill it. (wear gloves!!). A slow-setting CA might be best for this.
A well equipped hobby shop or wood working shop will have the appropriate CA glues. Products like Super-Glue have thickeners and won't combine with the wood fibers. The advantage of making your own filler is that the color will be near perfect without resorting to stains or other colorants.
The neat thing about CA glues is that the piece is ready almost immediately for whatever else you need to do. It's also partially reversible with de-bonder if screw something up, or someone else needs to repair the repair in 100 years.
Also, regarding finishes, note that if you use Watco, or tung oil or similar products for finishing, pay close attention to the label directions concerning handling any rags used to apply and buff it. If you wad them up and throw them in a corner, or a trash can, there is a very real and substantial danger of the rags spontaneously combusting. The organic oils can make considerable heat if they oxidize in a closed space.
Rags should be spread out on a non-combustable surface to thoroughly dry before discarding.