battery extension is ok.

I have 120ys (31.5 oz) and dual mini's in the back for the elevators, a micro way up front against the firewall for the throttle and the rudder servo in the rear tray. lots of wood/aluminum removed and the 4v battery about half-way between the elevator and the rear tray. balance dry at 4", tank is close to cg but still a little forward. all up dry weight is 9 lb 3 oz...I don't know how anyone can get much less with a 120 size motor...but...apparently they are !!

ys 91 = close too 27 oz not 24 oz

I would switch to dual servos on elevator in the rear and no added weight...If I were you !!