Do you want to just add a hard tail rear section or make a whole new frame? Whats wrong with your current 650 forks? Do you want to build the neck height to match the fork length or buy forks that fit your neck height? Planning on raking the front end or leaving it close to stock. Got a lathe? Lathe and a section of tubing and you have a new neck. Still going to have the issue of VIN numbers, you could always transfer the numbers off your 650 neck although its not quite legal.
Sounds like you need to do some more homework first, or we need more info.
Want to do it, or do it right...