hi, i did this last year to mine, cant remember the exact sizes but; don't step down the size, measure the breather outlet on the oil filler. go to your nearest hydraulic/pneumatic/flexihose supplies and buy a fitting that's barbed one end, the size of the breather fitting on the filler, and threaded the other in a metric thread and get a nut and washer to match. then drill a hole the size of the thread and push the threaded end through and then fit the nut and washer on the inside of the base of the filter with some permanent loctite, you don't want that sucker coming undone. my filter's carb inlet hole is offset so i placed the fitting in the wider space and on the bottom of the filter then ran a piece of codan car heater hose from filler to filter, this is thick hose so it won't fold when formed in a tight curve. my hose joins the filter just above the gen/alternator.
note; the filter housing is pretty thin so try to clamp some wood either side and then drill through the wood to stop the metal from twisting.
if you have an aftermarket oil filler you could simply take the hose from the breather to the hole in the tinware where the metal pipe used to be, if you use a grommet then the tin won't cut through the hose.
i would recommend the filter mod though, it worked a treat and looks like it's supposed to be there. you could always get your measurements then get a stainless fitting from fleabay for a couple of quid like i did.
sorry about the grammar but my shift keys have stopped working. exclamation mark.
cheers, dave