Just came across
THIS REVIEW of Revell's recent (I think) re-pop of the Viking ship. The decal sheet looks rather interesting.
Now, as you can see, each shield design is included twice. Get two kits and - hey, presto - four roundels for your Viking aircraft! Okay, for six roundels you'd need three ships.
Or find someone to swap.
There's a hole in each of the decals for the shield boss. I reckon you could paint the hole in a metal colour of choice, arguing the Vikings replicated the shield boss in their roundels.
Maybe the woodgrain drawn into the decals is a bit of a put-off, but again you could argue they simply wanted to replicate the shields.
EDIT: Forgot the scale! The ship's 1/70, so I figure the shield decals should make decently-sized roundels in 1/72.