Excellent decalling, especially on the cowl..
For what it is worth, my method of working over decals is first of all to cov er with a couple of coats of Klear/Future.. This can be sprayed or brushed, and not only seals the decals but also provides a barrier between the decal and further application.
Then I spray with satin varnish from a rattle can. This takes away some of the gloss, but isn't too flat. However there is then a very slight "bite" to the surface. I then dry brush pigment powders or powdered pastels for general staining or exhaust blackening, rib highlighting etc and the pigments then take hold on the surface.
Occasionally I will dry brush over the decal, the same colour that is underneath to show wear and tear. Literally tear, as I have had to hide damage to torn decals before!
Finally and only if necessary, a further coat of Klear or Satin varnish to finish and seal the weathering..