Like most Suzukis, the GN400 shares about half its parts with other bikes. Non-motor stuff can usually be found in the GS line, drivetrain stuff in the SP line, obviously. For instance, I recently swapped my stock 16" rear for an 18" from a GS450E.
They're kick-only so if you like a starter button, move along.
They also have a fairly high steering head and long forks which makes them poorly suited for a racer but a very stable all-around bike in general. Shortening or swapping for shorter forks starts to trim your trail very quickly.
The CDI's are starting to go and NOS items are stupid $$. There's a guy on the Yahoo GN site who's successfully swapped out the stocker with one from a Honda. Join the GN400 Yahoo group if you haven't already.
-Deek