It's not always that simple. In some states (including my home of Missouri), a VIN is only legal as long as the vehicle remains "substantially and recognizably" the same as manufactured. In other words, if it's a Suzuki 600 motorcycle, a cop better look at it and say, "Yea, that looks like a Suzuki 600 to me." Once you modify it to the point where it is no longer recognizable as the vehicle you started with (and changing the number of wheels is pretty much a sure dinger on that one), you may be required by law to get a Special Construction title, even if the original VIN is still present on the frame.
Be forewarned you might be required to provide a copy of the bike's original title (front and back!) for not just the frame and motor, but EVERY non-new part on the bike! For new parts, you must have an itemized receipt proving you bought it new. At least that's how it is in Missouri.
And the worse case scenario is, no matter what state you live in, if it looks like you incorporated the head stock into the frame specifically to maintain the VIN stamp; then you can get hauled in for VIN tampering, which is a FEDERAL felony.