Hi,
The main thing you have to scratch build is the cockpit floor.Then you should add two semi-cross-sections , one at the beginning of the floor and the second one at its end.I have marked these with arrows of different colours.Also marked the place where the pilot panel should be.
To begin with, the first step is to clean up the cockpit interior with sanding.Simply you have to sand off all unwanted thinkgs there.The inner cockpit sides have to be smooth.What is more sanding will make them thinner.So be careful of the sanding.Also it would be nice if you could have drawings of Zero cockpit in order to see how it looked like inside.For the floor you can use a phone-card or another thin ( max. 1mm) piece of plastic sheet.These cross-sections can be made with the same sheet.If there were these in the real plane, make them as the same in its shape.I mean all holes etc.But first of all, make them as a solid ones and fit to the fuselage.When the floor and cross-sections are fitted, mark places where these will be glued to the inner cockpit sides.Then add some stringers and semi-cross-sections to these cockpit sides like it was there.Then add the rest of scratch-built pieces mounted on inner cockpit sides.I mean the throttle, radio-remote control device, some wires etc.... When done glue the floor with these cross-sections to one of halves of the fuselage and paint.On the second one paint the scratch-built side as well..
The next step is to make the pilot panel and rear parts of the fuselage MGs that were seen in the Zero cockpit.Paint them as well and mount to the half with floor.If you want you can make some of weathering to the floor and other parts at the stage.Then put these halves together and you almost have the cockpit finished.The pilot seat and stick can be paited separately and added later.