super cub
Okay. First, a 91 4 stroke should fly it fine. May even be over powered. (I have an 81" Cub that flys well with a 52 4 stroke.)
Did you get the cowl with the plane? Put the cowl on, then using a pencil lined up in the center of the front cowl opening, make a mark on the firewall. Should come out close. (Or make diagonal lines between all those 'opposite' holes on the firewall. Where most of them cross should be the center.)
Start with a CG that's 25% of the wing chord where it joins the fuse. ie. If the chord is 12", place the CG 3" back from the leading edge (on the bottom of the wing).
I'm sure others will have different opinions, or better ways of tackling your problems.
Good luck,
Dennis-