I don't know what the airbox on a Rancher looks like but EHS racing makes a really nice airbox product. While they don't make one specifically for your rancher, they have a few different universal sizes that you might could make work. Check out
this page, in the bottom right corner of the product table for the universal sizes.
If you added a pipe, fuel controller and opened up the airbox with a proudct like the EHS one or the one that Battlegun noted below, you should see some gains in your quad's performance.
Buster