I have Simplicity 616, 627, 727 which are from the late 60's to early 70's, the first 2 still have their original engines and still work great to this day, the 727 has been repowered at some point in it's life from a 7hp to 8hp with an engine that was made sometime in the early 80's which is what the numbers off the engine showed when I looked it up.
The 616 has been in the family since new and the 627 I bought used a few years ago and looked up the numbers and it's engine was from that era when the mower was made which leads me to believe that it is the original engine as well.