I have a JD LT 160 that I believe the timing has been disturbed by a backfire. It runs but is rough and has little power. When I engage the deck it almost dies with power loss. Where can I find a manual that shows how to adjust timing?

THe timing wasn't disturbed in the way you are thinking. Likely the woodruf key on the flywheel sheared and the still tight flywheel shifted on the crank beyond its intended location.

If that's the case the damaged key needs replaced, the flywheel will eventually shift to a point it won't run at all if you don't.