Shocks.
Make it handle better at the speeds it can do. I would also get better tires - but they may not be an issue for everyone.
Brake lines and brake pads.
Make it safer at the speeds that it can do. Increasing your visibility would also be good. Majic blinkers will turn your blinkers into running lights that blink off to signal,
I wouldn't recommend doing anything to make the bike go faster before making it able to handle it better.
