Had the bike been sitting long before you bought it? It most likely needs a good carb cleaning. It would also be a good idea to change the fuel filter if you have one, if not I’d suggest installing one. As for the oil leak, from where you describe it, it sounds like a valve cover gasket or head gasket. On these older bikes small oil leaks are very common. If the bike has sat for a while, the seals have probably dried out some. Often just running the bike regularly again will cause the seals to swell up enough to stop leaking, if not you’ll have to determine exactly what seals/gaskets are leaking and replace them.