I can see where you are coming from and it soooo tempting just to do a little DIY work on your plane. But the basic reason you and your friend are legally prevented from working thinks like avionics, instruments, flying controls, seats etc. is that you don't have the correct manuals, training, testing, equipment etc. to perform these tasks. That is why you have to send them off to specialised repair shops. There are hundreds of smoking holes in the ground because some clown has "fixed" something on their plane. Worse, one of these idiots may have "fixed" something and then let someone else fly it who has no knowledge of the work perfomed. By all means work under the supervision of a "tame" licenced engineer, but please, don't DIY.
If you can't afford to have things fixed properly, you can't afford to fly. But more importantly, the people on the ground beneath you and those whom share the sky with can't afford you flying either.
PM