It it's a carby model then the first thing I'd do would be use the drain screw on the bottom of the carb and catch what comes out looking for water, then I'd turn the tap on and make sure plenty of fuel was getting right through the tap and filters and into the carby at a good rate. If you have a timing light you could connect it to the bike and watch it when the bike falters looking to see the spark wasn't dying out. If you don't have one then... If draining the fuel and checking the flow didn't change things then I'd take the carby off and strip it right down, entirely, and clean every jet and passageway, and then check the seals where it fits onto the engine as I was putting it back together. It will probably be right after that.