Eddie,

Your post is way shorter than mine. Don't be sorry at all unless you write like me.

I believe the YS 91 should be fine for the plane, just try to minimize the weight. The Acro version weighs around 3500grams (I believe this weight is with a YS 91).
FAI version is heavier around 3600 ~ 3900grams. Not sure how it got heavier.

Both versions have same wing area of 56.1 Sq.dm.
Since the weight of final RTF plane will vary depending on the equipments and engine used, as long as you are under 3900grams, you should be fine.

My Beat On 90 weighs around 4100grams with current heavy wing. (wing weighs 1160 grams). Wing area is 53.0 Sq.dm.
It flys ok with 30% nitro, so Explorer should not be a problem.

For the engine mount rail. How thick is it? I don't remember how thick it was in my Explorer Acro. I also do not have picture of it.
For reference, I am attaching a few pictures of my Beat On engine mount. The rail is 3-ply aircraft plywood under the actual mount. (about 10mm thick)

Additional ply-wood in the front end and rear of the mount for installation of the engine mount.

Let me know if this can help. If not, I will try to take a few more pictures. (My Explorer is behind a whole room full of boxes.)