We use a 26mm OKO for our 349 and 348 Cotas. The kits include new manifold rubber sleeves, extra jets and for the 349, a sleeve for the air box. These kits improve mostly low and mid-range power.
Our OKO kit for the Cota 247 uses a 26mm carburetor. The kit comes with a flange adapter and a 10mm thick heat block that replaces the longer Montesa heat block. Extra jets are included, as well as a short 2"ID sleeve that goes over the carburetor intake for a better fit of the air box hose. This kit is a big improvement over the stock AMAL.
Our Cota 123 kit uses an 21mm OKO jetted for it. The kit also includes a flange adapter, a 10mm thick heat block and extra jets. The air box hose will fit the OKO's intake. We found the back of thetop of the OKO may hit the frame. Since the flange adapter is rubber, the heat block is not needed and can be left off if frame clearance is a problem