I don't have a lot of time to go into it, but the basic idea would be to scan the decals at 300 DPI or so. This is laserwriter resolution, so you will have a high res master to work from. Save that somewhere and always open it and then do a SAVE AS to generate another working copy while you work your way along.
Photoshop has a nifty feature known as a "Magic Wand" that will select a range of pixels of a similar color from where you click it.
Use the magic wand to successively select all the pixels of the surrounding area, using a bit of anti alasing to creep a tad into the edge of the decal image.
Once you have all of the space around the decal (and any holes inside it) selected use the "Selection" menu and choose "Inverse" which will swap the sense of the selection and you will no have everything that IS decal selected.
(Might be a good idea to save the selection to a channel about now and name it)
With the decal selected, do a Copy.
Then open up a NEW file, addint 10 pixels to the numbers that come up, which are usually based on what's on the clipboard at the moment.
Make the layer transparent that you are pasting into.
Paste the decal image into the transparent layer.
Save that state.
Do the math on what size you really want by say, 150 DPI which is fine for an inject and scale the decal. That should shrink the image which is the direction you want to go. Do the image touch up and perhaps do a light 'Unsharp mask" to crisp things up.
Save that state.
Print an example on plain paper.
You should be pretty close.
When you like what you see, print to decal paper.
The transparent pixels in the document will be printed in transparent ink on the decal paper and all you will see will be the colored inkjet droplets combining to be your decal image.
Spray it with the clear coat discussed on previous threads.
Apply and enjoy.
Cy.