Create an event (wherever you want) called
EVENT_DISPLAYREST and write there your code for the new hud.
With
rotatesprite you can place the decorative sprites. For getting the current ammo, save first the
curr_weapon value inside a var, and then use that var along with
ammo_amount to get the current weapon's ammo. To show it on screen, use
digitalnumber.
If you manage to do it, then use
got_access to determine when to draw the small key icons.
I hope I have explained it clearly. If not, feel free to ask whatever you want.