Re the engine. Yeah, it's kind of small, and I don't know how much I can physically do to it. We'll see.
And speaking of which, in the diagram posted, the ignition wires are shown connected to the right of the cylinder, while that museum example has them on the left... does anyone know which is correct? (I have been unable to find a photo of Babs showing the engine in place clearly)
As memo serves the engine for the Ki-15-I "Babs" was the Nakajima NAP (Ha-8) but not the Nakajima Hikari 3. In the pic below it can be noticed that there were two cables going to the two ignition plugs. One cable was going from back of each of the engine cylinders to the right spark plug while the second one was going from bottom to the left one. What is more the wire coming form the back was running in front of the tappets for the Nakajima Kotobuki III engine used for the Babs prototype. The Ki-15-II was powered by the Mitsubishi Ha-25-I engine. The Navy C5M1 was the improved Ki-15-I while the C5M2 was the the C5M1 with the
Nakajima NK1 Sakae powerplant.
the pic source: the net.