Is the fuel pump working? I bought a non running YBR125, the engine check light came on and went out after 5 seconds as it should. This showed that the ECU and sensors were ok but you should also hear the fuel pump running during those 5 secs. I had a spark at the plug but it was dry with no signs of petrol getting through. I replaced the fuel pump and all was then fine.
The on line Haynes manual is very good and if you have a multimeter you can check all the relays etc. You can buy replacement fuel pumps on eBay but they are a bit of a fiddle to change as Yamaha did not design the pump assembly to be serviceable.