RE: OS FS 91 is it capable ?
With a 70" span I think you're right on with the .40 four cycle. My father has a 72" Buzzard bombshell with an OS .40 Surpass, and that plane is WAY over built. Almost like they carved it out of a solid block of wood.
Depending on the model and how lightly you build it, you might even go with a smaller engine. I've got a 67" Quaker with an OS .26 Surpass, and my father flys a 70" span Playboy on a .15 two cycle.
The best flying old timer I've had my hands on is the Dalaire Sportster. Penn Valley Hobbies sells a decent kit for it with 108" span and as I recall, they recomend a .52 four cycle or so. The build took my dad maybe a month. He hung a .70 Surpass II on the nose of his, and it is quite a bit over powered for an old timer design.