What is the reason for narrowing the building?

Doing so will detract from the building in my opinion. The work has already been done with the exception of unknown repairs. To narrow the building will involve changing the tie beams which looks like they have steal added already. But more involved would be the rafter system. I would use its original size. Depending on the joinery used or lack of joinery it would add up to a fair amount of work. If the feet and peak are just butted and spiked, might not be too bad. Build new, to the size you want? Looks like a simple straight forward frame.

Pay close attention to the tie to post connections.