What do I do with the crankcase vent hose?
The crankcase breather tube is an s-shaped tube that sticks up from the crankcase below the carbs. It is normally routed to the airbox, so case gasses can be taken back in through the engine intake and blowby oil reabsorbed to the sump when the mist accumulates and drips back down into the case. This is the tube; filter manufacturer is unknown.
With the airbox gone, this vent is unchecked and the crankcase has no protection from the elements, except for the s-shaped hose. Also, if the hose is not otherwise dealt with, then the oil mist from the engine will cover the top of the case and the bottom of the carbs. The hose could be plugged, but then the case would be pressurized; not a good idea.
There are two practical filter options: buy a permanent, cleanable filter (looks like a small K&N pod), or get one with a replaceable element. You may want to do the latter, since cleaning the crankcase filter needs to be done much more often than cleaning the carb intake filters.
Select a compact filter with a good-sized surface area, such as part # B23124 (Purolator), BA3749 (Fram), or BF305 (Deutsch). These filters are all the same, just from different manufacturers. This filter has a terminal connector of 1/2" O.D., just like the breather tube; use a 1" length of 1/2" I.D. hose to couple the parts. The filter has a low profile and fits beneath the carbs. This comes with a replaceable element. Buy the replacement and replace with every oil change.
We have below a set of pictures showing the following parts, found at Pep Boys:
As with all mods, there are some compromises. The breather will be under the carbs and behind the bodywork, but if you're worried about water getting in, turn the opening down. If you're worried about oil getting on the engine, turn it up.
Something else you might try is taking your breather hose into your local auto parts emporium. Look for a crankcase breather that will fit the hose. You want a crankcase breather that's small enough to fit in the assigned space.