1: If you can lay the original panel over the gun bay, tack with pva/elmers, but make sure there is no pva on the surface.
Carefully lay decal over the cover & wing, (do not use solva set). allow to dry.
Then carefully slice the panel lines, peel off the the pva with a cocktail stick, & solvaset the decals afterwards if needed.
2: If you have a Tamiya kit, get the markings/decal layout sheet, copy it, lay decals in the correct position on the layout sheet, & carefully cut the decals.
Otherwise enlarge the markings sheet to the size of the model, & as #2.