What do you have to do to make a 2007 YAMAHA WR 450 F CA street legal?

You need a good friend in Colorado. In order to make a WR450F street legal in Colorado you need to add a Baja Designs kit that includes turn signals, mirrors, speedometer, odometer, horn and DOT tires. In 2003 the emission law in California was changed such that no dirt bike can be converted to street legal. But you can buy a 49 state street legal vehicle and register it in California if it has more than 8000 miles before you bought it or you moved to California with you vehicle. BTW there is a new model WR250R 2008 which is California street legal.