If the break is the result of too much force, and not due to decay in the wood, it can probably be very adequately repaired. To do this you need: a thin bladed tool such as a narrow putty knife (in fact a couple, of varying widths might be good); several clamps that will fit in the space you have to work, but provide good clamping pressure; a good epoxy with a relatively long working time - at least 20 minutes or more; packing tape; acetone or ethanol (for cleanup); a couple of shop rags.
First, you want to work the crack open a bit more with the knives. You want a crack into which you can confidently work a thin epoxy as fully as possible, but you also want to be careful not to fully break the original wood, so work carefully. Then tape the bottom of the crack (to prevent the epoxy from all running out) with the packing tape. Now in a warm place, with everything warm (minimum 25C), mix the epoxy. You want a relatively thin, flowable mix, so the warmth is important. Don't be afraid to warm the resin and hardener in a hot water bath before mixing if neccessary to get this. A throwaway trial mix is worth the time, if you have doubts. Work the epoxy into the crack by dripping it in, and working with the knives, until you're confident you've got the crack filled as thoroughly as possible. Clamp the piece to bring the crack together well - doesn't have to be super tight, but the crack should be closed, and epoxy should squeeze out all along the crack. Wipe away excess epoxy on the outside after clamping, then wait until the remaining residue has become quite tacky, and use the acetone or ethanol to clean that off the piece. Leave the clamps on for 24 hours.
If you work carefully, you should end up with an almost invisible repair that is a strong or stronger than the original wood.
