RE: motor mounts
Servo's will come included with the radio system when purchased.
As far as a rudder and other components, there are no commercially available parts for munc of the stuff we use. Some small items are used from the model airplane community while the vast majority of parts are home made.
The motor pylon mounts dadaddyro mentioned are very good and are used by two different manufacturers for their kit boats (AquaCraft and Dane's Hobbies). Besides the nylon mopunts, you will have to make or purchase the wooden platform that the engine sits on. This is also anAquaCraft item that can be purchased from Tower Hobbies.
If you go to the listing for thr Air Force, you can get to a page that lists replacement parts for this particular hull that could be used for a hull of your own design.