It is very easy to print your own decals. You can use Word to do lettering and numbering if you want. A good graphics program like Photopaint or Photoshop will be helpful for more complex stuff.
If you have a flatbed scanner, try scanning the side of the car/engine you want to make decals for, this will help you get the correct sizes. Be sure to scan at a high resolution, 600 or better so your decals look better.
A good photo quality printer is also helpful. You'll get better decals with a good printer. If you are using an inkjet printer, you'll have to seal your decals before applying them. Any oil-based clear coat will do. I use testors from a spray can. I have found that Epson DuraBrite inks work best for me, as they are semi-water resistant, so they don't bleed when sealed. My HP inks tend to get a little fuzzy when sprayed with clear coat.
Unless you've got an APLs or similar wax printer, you can't print white, or any light colours. If you print yellow on the clear decal paper, it will look great, but it is not opaque, so if you apply it to a dark surface, it won't show up. If you need to print white, get white decal paper and print the back ground colour around the white. If you match the paint colour well enough, you won't be able to tell.
Have fun, and let us know of any tricks you uncover!
