What I do is create a plain number the way I like it with the correct font, size, etc. Then I paste the engine-turned pattern over it as a new layer. Using the "magic wand" tool I select the number shape (you may have to turn off the engine-turned layer so you can see where you are selecting with the wand). Now I make sure I have the engine-turned layer selected in my layers window. Hit Select-Inverse so I now have everything in that layer selected except the number itself and hit Delete.
You should now have a layer with the number the way you originally created it and a layer of the engine-turned pattern in the shape of the number. If you want to make sure they stay lined up together while moving them around your car you can Merge Layers or Lock Layers.
Did that make sense to everyone? Sometimes I know what I'm talking about, but not how to say it. ;D
