In addition to the roundals, you need the registration number. To make a decal with that, you need the right font. It has been awhile, but there have been discussions in some of the forums on obtaining the fonts for both civil and military fonts, and I managed to find both. Just cannot remember where. But the task is first to identify the font, and then do a search.
Once you have the font you can use either your graphics program or your word processor to do the decals. I find a graphics program easier to size the decal to what you need.
You might try going to the decals forum and ask about the font.