Moondine

I know someone can bail me out here... I´m purchasing a bike (1981 yamaha xj750 seca) from my neighbors son who now resides in California. Since his move he has lost all paperwork on it but I still need to get a title to plate the thing. To make an EXTREMLY long story short, where can I find the VIN on this bike? The VIN on the sticker has song since worn off due to cable chaff and I see no stamping for it. Any help here? Thanks, Moondine