Permatex makes MotoSeal for sealing crankcase halves. This is available at your local automotive parts store. Your motorcycle dealers also carry their own brand. Yamaha's is called Yamabond, I don't know the names of other specific brands.
Take a look at the microfilms for the bike. If they list a gasket, you NEED to run the gasket. Without a gasket, the main bearings will be too close together and damage could result. Chances are, there is no gasket specified. Also, if the bike doesn't specify a gasket, you cannot use one.