First Bihamby..don't take the instructions as the bible building this plane..it is wrong in many areas.
Those horns are meant to bolt on to any of your existing plastic horns with those little tiny bolts and nuts they give you. Personally, I don't like plastic, especially on a 50cc plane.. with high torque servos. I would invest in some nice metals arms for your servos. (I prefer AirWild)
Use common sense when building this arf. I upgraded most of the hardware to stuff I trust..i.e. better tailwheel assembly, titanium axels, nicer tires, metal arms. I did use the ball link hardware as it seems quite nice and strong.
Add extra support/glue around the landing gear attachment plate inside fuselage. I also doubled my engine mounting plate..the one supplied was hardly 1/4". I also pinned the firewall and angle stock inside and outside the firewall. Do not permantly mount your batteries, until you finish the plane..then use the batteries to locate
the proper cg. Mounting the cowl is a cluster *&^%. Also you should add a support brace or backing to the inside of the front of the hatch, where the carbon fiber rods go in. You should also added extra glue to the back brackets on the hatch. I added extra balsa support to both sides (inside the hatch). If these brackets break..you hatch will be a flyin!. The hatch is very weak plastic in front (red part) and any pressure downward will crack it. You will find a strange arrangement of 4 pieces of wood that fit all together..to form a brace for the top of the firebox..and it will support the top of the cowling..Of course you won't find info or instructions in the manual for this!! Measure your engine and determine what distance your front plate needs to be at for proper cowling/prop clearance. I sure I will find more things to worry about with this arf..as I build along. Good luck with yours..and if doubt about something, just post up your question(s).