Here's a link to an air filter adaptor that should fit that engine, the ad has bolt pattern measurements so you could make sure it'll fit:
http://www.bmikarts.com/Air-Filter-adapter-for-Tecumseh-Briggs-OHV-Engine-_p_463.html
I get the feeling that kart is one of the lower-quality racekart models made in china. There's been quite a few reports of those having problems with frame welds failing, plus problems finding replacement parts, like brake calipers and spindles...
One easy way you could tell If its one of the Chinese models is to ask the seller if it has a steel motor mount that's welded to the frame, or if it has an aluminum motor mount that clamps onto the frame. Most all racekarts made in America or Europe in the last 30 years have aluminum/clamp-on style motormounts (like the pic I posted below), most of the Chinese made racekarts I've ran across have a steel motormount that are welded to the frame.