You will need something to hold the tube near the engine ,either a t bar or use a plywood one,just a slot in some ply glassed between the rails works.Its just to stop the tube moving really,silicone the tube in place at the transom to seal up the hole and fix it to the plywood at the other with silicone .A zip tie at the motor side will stop the tube moving back till the silicone sets and its extra insurance too i suppose.Just remember its easier to take out if required if you use silicone or a t bar setup rather than glassing the tube in.What are you using for servos? you will need a high torque one for the rudder,standard is ok for throttle.Im asuming your making your own mounts for the engine? ,be sure to use rubber dampers if you are.What motor are you using?,the collet might not fit your output shaft? ,you can use a square ended flex if your using a trimmer type engine that has the square hole?.If not you will need to find a collet with the correct threads,i used a cmb .90 colett for one they have the m8 thread same as my weedy engine had.My first one had the square setup so then i had to use a ferrule to attach the cable to the shaft at the prop end using silver solder,you can also use loctite retaining compound for this.
Mart
Heres a pic of a typical ferrule setup.