USS - If it's a 1921 Royce, then it's not a Phantom II. If it is indeed a Phantom II, then it would likely be a 1931.
I have a number of Phantom II models in both 1:24 and 1:16 scale. If you can get me the engine number of the plate mounted on the car's firewall, I can tell you exactly what model it is, who did the coachwork and which model you will want to use as the basis for your replica.
You'll also want to check a couple of things in terms of whether or not it's a PII Continental (if it is, they'll be an exhaust cut-off switch on the floor by the driver's feet and whether what kind of air cleaner it has. Also, is it right-hand or left-hand drive - that's going to be important for obvious reasons.
If it is truly a 1921 Royce then it's a Silver Ghost and that's a beast of an entirely different color.
Let me know. I've got a stack of Royce models in my workshop that I'd love to see someone build.
Andy.