By 'air vents. I imagine you mean 'cowl vents'. If you Google this you will see lots of full size examples. They are readily available from model boating suppliers in different sizes. Some are expensive but you can also get cheaper plastic ones such as these from Reade Models **LINK**
Cornwall Model Boats also have a large selection: **LINK** just enter 'cowl' into the search box.
Although not always shown on the modelling examples, the vents often have an upper section which can be swivelled to catch the wind or not as required. Sometimes this is by means of handles mounted on the vent and sometimes there is a gearing mechanism which enables them to be turned from the accommodation below.
Obviously you can cut the vents down to size or make them taller by glueing a section of tubing on the bottom.
Hope this helps,
Colin