Timber for step

During the process of lengthening the doorway to fit a new screen door I busted the hardwood plank that lies across the base of the doorway (the threshold?). Pulling off the threshold pieces I noticed the plate underneath is slightly rotten in places, as is the floorboard adjoining. Short of burning the place down (crossed my mind momentarily), can I clean the rot and bog before placing the timber step over the top? Can I use pine for the threshold? (treated pine is $11 and hardwood is $41) And what should I use for bog? And, come to think of it, can I just bog the gap where the threshold used to be and leave out the timber?

As usual I have turned an easy job into a complicated one. :doh:

thanks in advance. Brett