Hobby King sells velocity stacks for $2.00 US. If you fly from a dusty or sandy area, a filter is not a bad idea. I have a large piece of old household carpet I use for starting and running up my engines, but was still seeing some cylinder scoring from grit injested during take offs and landings so I have fitted air filters to some of my engines. (Very sandy where I most often fly.)

https://adaracing.com/parts/hobby-rc/air-filter-kits

AV8TOR