Em, engines that I have had on my 4Star60: OS61FX, OS91FX, SuperTiger 90, Saito 91, Saito 100, YS 110, I use it for an engine run-in (or fly-in) test bench. I used different Hayes AL60 long mounts which allowed me to use one mounting pattern on the firewall and switch between the engines by simply swapping the mount for a new one. Yea, had 2 different throttle control rod approaches - no big deal.

Reinforcements?
Tri-stock on the back side of the firewall.
Tri-stock between the landing gear plate and the fuse sides.

Alterations?
I added a tank hatch on the foredeck.
Remove 1 or 2 wing rib bays to improve the roll rate of the 4Star.
Cut down the bubble that SIG calls a canopy. Saw a 4Star60 today that had a dark tinted Heli canopy - very nice.
Some would cut the amount of dihedral in half.

3)- yes. You can use CA for a quicker build and you can better ensure location of the joint because it dries almost immediately. Doesn't sand well at all!!!! With Titebond you'll need to pin, tape, clamp, weight the joint to ensure that the joint remains fixed during the drying cycle. If you are gluing a butt joint with end grain, pre-treat the end grain with Titebond to "satisfy"the end grain absorbtion of glue. Then use just enough glue to wet the surface. MinnFlyer told me to apply the glue then just wipe excess glue off with my finger - what remains is enough glue for a strong joint. If you wanna get anal (or are planning to use transparent covering) use a cottom swab wetted with water to clean up all around the joint.

HTH