The airplane was originally designed for a 60, not a 45. It's also a biplane and they wind up using more power at low speeds because of their extra drag.

The 70FS would be an engine that would pull the plane, but lots of people wouldn't use the term "good".

Mine flies good on an OS61FX.

I've got a few 46AXs and an OS70FL. They perform about the same. I've never even considered putting either of them into my Skybolt. And now that I have considered it, I can say without reservation that I wouldn't put either in and expect good performance.

But your 70FS engine in WM's 40 Ultimate would be a good combination.