Norvil used to sell this part. I suggest you contact them directly as it's not available through their on-line purchase system. You can search their site using their description
GAUZE - SUMP FILTER - LIGHTWEIGHT TWIN - ALL MODELS
Strangely my search by part number failed to find it.
They have a lot of Lightweight parts if you have the part number and send them an enquiry.
Duncan.......Norvil have got a whole shed full of Lightweight engines, Including loads of Navigator motors. They are bound to have some second hand filters. But........you will probably need a cheque book in each pocket.