Hi MickyD
Could be the cause of your issue by the sound of it, maybe see if you can google an image of the cover to see where the vents are and replicate that making sure that the air entering the box will travel through the filter element. Can you put a fitting on the cover so you could run a flexible snorkel hose up high on the handle bars ? Sorry I can't be of any more help as I'm not familiar with that model. If it is a Sanli I'm sure a cheap one will appear on gumtree that you could rat parts off, plenty of them would have been sold.
Cheers
wce
