The side covers are a very distinctive feature of the Off and will be fun to model. The crankcase breathers are afixed to the top. I start by importing a picture with a side view, scale it and overlaying the outline.
Here is a side view showing how the middle is raised above the edges and the fin detail. The side cover and breathers are one of my favorite features of the Offy.
Then take a shot at the swept vent feature.
Then begin to lay in the cooling fins. There are a total of 9 on the Crankcase cover.
And the Block cover plate in yellow. I also worked a little on the timing gear tower and added the housing for the bevel gears that drive the magneto. That is the extension on the very left middle. I also cleaned up the front cover. The magneto is mounted atop the small shelf at the top of the front cover.