When I bought my Flash "replica" in 1993, the fork blades were milled only the lower part, inside. I have been told by experts that blades were prepared in different ways, and this way (pattern and depth) seems to be one of those. My blades are done by an (unknown) expert. I had the blades without paint a few years ago, and there was an absolute flat surface, needed no filler or other cosmetics.
I have used the bike solo as a racebike, and for a few races even with a sidecar mounted. Since 2002 it is used as a road bike, sometimes with a (light) sidecar. Now the engine is a 39bhp big bore and is used very frequently, sometimes two up. So in my opinion there is no problem when you mill the blades not too extravagant. I have no experience with milling the top part...