Don's example is a good one. Doing something to resist wall spread is something to think about. Perhaps this was originally designed such that the weight of the roof was considereed low enough relative the wall stability that they didn't tie the walls? Of course everyone will feel better having a tie beam.
One thing that I can't tell from the pic is how the collar tie is attached to the rafters? Was it just nailed up? Is there mortise and tenon and pegs? Calculations could be done using the dimensions, and estimated loads to estimate how much force is being applied to the top plate from the rafters. This would help you size a tie beam and how many and what size pegs to use. Depending on what your intensions are. You could also use a steel rod as a tie if you don't want to go to the truble of a tie beam and don't care about historical accuracy.
