http://www.nsr-world.com/workshop/

Have a bit of a read around the site, heaps of info. I'd start with the basics - do you have spark on both cylinders? New plugs might help.

Check your coils, clean all the electrical connctions - search for how to test coils there was a really great thread with lots of pictures just recently.

Pull the carbie's off and note the jetting. Give them a really good clean.

Test the compression and search for leak down tester if you want to check the seals are in good condition. Hard starting is a sign of a dodgy seal. There are lots of threads dealing with how to check out the crank seals.

I'd do this before jumping into a rebuild. That being said, a new top end would not go astray, especially if you don't know when the last one was done.