Try a sign maker for screen material. You'll need polycarbonate, 5 or 6mm. You could also try e-bay, as there's loads of off-cuts on there.
Have you dropped the gear oil, or tried a pressure test on the casing? Is it gear oil that you can see on the shaft, or residual oil from the 2-stroke exhaust?
BTW, thread titles such as 'Advice Needed' hardly draw peoples' attention to your particular problems. There may be experts on screens (eg Nugent) who would overlook the thread, due to the ambiguous title.