The thing is you don`t need to bolt the air filter to the engine,it could be fitted lower down in the hover in a cleaner air position and linked to the engine air intake with ducting.
There are loads of after market parts available out there , a lot of them are not designed for our type of environment
In the real world machines designed to work in dusty or wet environments have good quality filters fitted to prevent down time on the job, I don't think hovercraft's are any different