Depends what you want to do to it and how much you want to spend.
The easiest mod is to open her up, smooth down and polish the internal moving parts and the inside of the shell.
Next you can upgrade the internals
Cyclone feed & internals, response trigger, e-grip, zero kick hammer, new bolt & power tube. The list goes on good sir.
I absolutely love my sierra one and has been fully upgraded including e-grip, the gun is great fun! It does take time and money but if you like the gun it's worth it