Honestly, I think you'd be just fine stamping the VIN on the frame vs. replicating a factory metal plate or decal. My Yamaha XS650 for example has the VIN stamped on the steering tube. I don't think they had industry-wide standards for these labels or the requirement to have one.
If you're doing a full on restoration then it's worth bothering with, but you're not going to get a ticket for it not being there.