Thanks gents, I’m a few baby steps closer.
One of the easier changes, reducing the tailplane area a bit. Here the excess has been cut off from the starboard half for comparison:

After some sanding that should be passable.
The nose is trickier. Fortunately I can use the kit pieces, as their outer shape is fine. I just need to cut off the upper part to make way for the larger engine, and put a new, larger panel on the lower half. Plus some additional pipes/openings (carburetor intakes, sump oil, air scoops?).

The engine itself will be based on the kit: I will merely add four cylinders. I took measurements of the WNW RE8 and the dimensions of the kit pieces are ok, but my handmade cylinders are closer to the WNW. The kit shape is wrong, consisting of a lower straight cylinder and an upper straight conical cylinder, instead of a continously curved cross-section. Only the front pair will be visible, as the others will be covered with heat shields. I’m building this based on the fantastic images published by the vintage aviator on their Be12a replica.
The crank case (which won’t really be visible) and cylinders are separated so far, so I can turn it with the kit or scratched cylinders in front depending on which turns out to look better after painting. I’be added magnetos so far: inlet pipes, spark plugs (only the two that will be visible) and wires will be added later.

Observer’s office can be closed by simply covering it with a thin plastic sheet. The fuel (and oil?) tank will take its place, but only the fill points will be seen. Apparently I only took a picture of myself making a paper templete, but at least it shows another small change: closing the small skylight in front of the cockpit. edit: and a lot of yucky sanding dust in the cockpit!

The biggest problem so far has been the stitches going along the entire fuselage, holding the cloth panels in place. They are basicly non-existant on the kit. I spent more time than healthy to come up with a stamp to make a pattern that can be used on thin plastic sheet, that in turn could be used to ”skin” the plane. I gave up that idea because stamping strips gave a raised look which I didn’t like, and skinning the entire sides I don’t feel like doing. Two stamps goes into the cabinet, maybe I get some use of them in the future. It took one full movie to drill the holes and thread them with wire (after marking the locations using the traversing table on the milling machine - NOT while watching any movies!). At least it is working, and if one is patient it shouldn’t be impossible to actually sew ”skins” together and glue the whole shebang on a fuselage, wires and all, to give a realistic look (if one has enough movies to watch). Maybe it could be also possible to convince the boss’s sewing machine to sew 0.3 mm holes on thin plastic?

Anyhow sanity returned and I put all those thoughts on hold and just scribed the dividing lines between the cloth covers and marked the holes for the threads - and rely on the brain of the observer to fill in the threads themselves abstractly. Put an oil wash in to better see the result. Not perfect but an improvement. Wobbly lines may sometimes even give a more realistic impression than straight edges and uniformity.
