I have a Lawn Chief Model 12-39F and need a diagram for the blade belt layout. The belt slipped off due to a loose pulley and I am not sure if I have reinstalled it correctly.

Hi,
That is not a true MTD model number. Look under the seat for the real thing, or try the link in my sig(OPE by Crigby.tripod.com) and see if I have it there.
Peace,
Clarke