FSX has controls for "Autopilot heading bug (increment)" and "Autopilot heading bug (decrement)" which operates the HSI heading knob and you can assign these controls to keyboard keys or joystick buttons. I use it with joystick buttons (with medium repeat) and that works nicely.

Unfortunately, the availability of FSX controls for different actions are in general scattered and not very consistent. For some reason the heading bug is the only knob (anyone please correct me if I'm wrong) that you can control directly using FSX controls. Most of the remaining knobs can only be indirectly controlled by binding a key to a "... (select)" event (if you can figure out which one it is) and then use the "Selection (increase)", default bound to plus-key, and "Selection (decrease)", default bound to minus-key, to adjust the knob. So, for instance, the course selector can be controlled by using the "OBS indicator (select)" control, default bound to ctrl-v, followed by plus or minus to adjust.

I hope FSX devs one day gets around to clean up this part of FSX so we can choose key bindings to operate each knob individually and, dare I say, perhaps even have controls that allow us to set auto-throttle speed and extend and retract the gear instead of just being able to toggle it. And while they are at it they may as well make a way for add-on devs to add new controls for special gadgets not found in the standard planes, thank you :)

[edit for typos]