If you can use a 3D modelling program (or willing to learn), you can build an FS4 aircraft, (one that uses steam gauges). It's as simple as that. Modern screen based aircraft are a little more of a problem but not unobtainable with some compromises.
Helicopters are more complicated but the TMD files are strictly kept secret so without IPACS say so, they are impossible to make as you need to earn their trust first.
I started with the F104G and lots of help from the one and only Master Obi Jan.
It can be done, it just takes time and a willingness to learn.
Steve