If the main stem broke, then the tree with probably shoot up a couple dominate leaders and the tree will have a forked form in time.
Black Walnut trees can stump sprout if they are under 30 years old but chances are slim for an older tree.
Walnut sawlogs are usually 16' long and at least 20" in diameter. Hardwood market is really soft right now, lots of hardwood sawmills here in Iowa have closed their doors.
I would just drop the tree and buck into firewood for future youth bonfires. Then plant a couple trees to replace it, probably choose a better tree. Where do you live I can help you out with a decent tree selection.