I'm trying to determine what would be the best 4 stroke engine for the Sig Something Extra that I'm building. Any ideas?

ys 63 or saito 72 or os 70

What he said, My bud has one he had a OS 70 in it and had power to spare.

Dauntae

Will the YS 72 fit the same width engine mount as a 2 cycle .46 or the Saito 63 ?

All of these engines have about the same footprint, all fit the same engine mount

The Saito 72 likes the APC 14X4 best, and that prop is one inch too long for the SE.
That leaves you with only the 13 in. props for choices, and not the best match for your combo.
Or go to a slightly bigger lgear like I did.
Terrific plane/engine combo you got there, wish I still had mine.

most 4 strokes are going to want a large prop, the SE has problems with 12 inch props and larger unless you replace the landing gear... the general recommended engine is the irvine .53, great 2 stroke engine for this plane, don't pass it up just for that. (it is almost as quiet as my saito .72 btw!!)