I'm guessing u purchased a hi flow pod filter of some sort and discovered the hose in question no longer fits anywhere so u left it open
And now it has fallen off
Firstly u must remove the valve cover and se if there is any dirt in there
Then either clean it best as possible or re install the valve cover
Then change the oil to be safe
Oil is cheap, engines ain't
Then but a length of hose that fits over the barb on the valve cover
Run that hose to a small filter of its own
U can buy a small pod filter that will suit that hose from a site selling performance bits
Or most people use a small inline fuel filter
That is a really cheap really easy option
U can't block that hole off as it is the breather for the engine
It keeps the inside of the engine at or nere atmospheric pressure
I'm playing with a quad today that has had a blocked breather for some time
Some of the repercussions are a blown out oil seal
Blown out gaskets in a few places and it has cracked the side plate from exesive pressure in the crank case
Also as it is the breather for the engine it must be filtered as any thing that can fit up that hose will end up in the engine
Like dust and water