Don't expect to get any noticable improvement in horsepower. Believe me, the factory engineers are not stupid and they don't throw horsepower away for no reason. There are a lot of gimmicks sold to people who are hoping for cheap, instant improvements but the bottom line is that they simply don't work.
Is some cases you can barter your power: trade some off the top to get more on the bottom, or vice-versa. If you want to start making major engine modifications you can trade reliability for a bit more power by making some moving parts a bit lighter. You can modify the compression ratio and get a bit more power at the expense of having to use expensive racing fuel. The cheapest and least trouble improvement would be to bore the cylinder out a bit larger.
Modifying your air box is not going to do anything beneficial for you.
The best modification you can do to your present airbox is make sure you have a clean properly oiled airfilter in place. That will gain you as much as opening it up and allowing more dirt and moisture in.
Air box mods are for dual sports and enduro bikes that are plugged up from the factory. The airbox on your CR85 breathes as well as it ever will in stock form. Focus your efforts on keeping your air filter clean.