I agree with the other advice. Cut the original blank to avoid as many checks as possible, rough it out, and then if there are checks, open them up with a grinder or handsaw, then epoxy the crack and pound a cedar shim (precut to have grain similar and running in the same direction) into place. One thing old timers did on cracks was to run a drill hole into the end of the crack, believing this prevented the crack from continuing to open up. If you do this (I don't), then fill the drill holes with pieces of dowel epoxied in place. Once dry, the cedar, dowels, and epoxy can be taken down and sanded smooth, making an almost invisible repair.
Any repairs made will be better and more stable if you hollow the decoy, relieving, as much as possible, left over stresses inside the blank. Generally checks are caused by either cracks caused in felling, improper drying before sawing, and poor sawing practices. There aren't a lot of huge cedar trees left in the upper midwest, so to get three and four inch planks the heartwood is often included in the plank. When the heartwood is part of the plank, the outer rings naturally shrink and dry at a greater rate than the inner rings, causing radiating cracks, usually starting at the outside and running inward. Hollowing the decoy can stabilize the large difference between the inner rings and outer rings of the grain.
Final tip: make sure if laminating two boards together to make the body, you match the grain so that any tendency of the wood to cup causes the two pieces to cup together, not apart.
Oh yeah, and tell the guy who got you the cedar to not give you pieces with as many checks in it in the future. ;-)
