How do I repair a Japanese Maple that I tore some bark off of?

Hopfully, the damage is vertical. They can recover from that pretty easily. Just prune off any ragged or dangling pieces of bark, and allow the wound to air-heal. You might spray the exposed part of the trunk next spring with permethrin to prevent borers from gaining access opportunistically. But aside from that, it should heal on its own. Do not cover it with wound gunk or tar. Experience has shown that it merely provides a dark moist place for critters to get under and cause mischief.

Japanese Maple Bark

Cut paste works wonders.

Smear it on the area and that's it.

This stuff will allow oxygen transfer, but keeps bugs out.

Bonsai guys use it like crazy.

Now go drink a beer and relax.

If it is for cosmetic reasons that you want to cover the area, then one way is to buy some pruning paint/sealer from a garden center or home improvement store.

It is like a black tar paint and it seals the surface and may help keep certain bugs out without damaging the tree.

The jury is stil out though on the value of pruning/sealing a tree wound. The tree will naturally create new bark to heal itself, so painting it would be mainly cosmetic.