My mistake right out of the box?

Hi, Two weeks ao I purchased a new 6.5 hp pull cord push lawn mower. As I was getting it ready to use, hurrying as usual. I made a big mistake. I put the supplied oil into the gas tank. I discovered my mistake when I went to add the gas. I drained out the oil . This removed most of the oil. I then added it correctly.After much pulling I got the engine to run, it smoked but seemed fine.A week later it ran for a while but lost power, then quit. I put in a new plug, no luck. How can I get my mower to work like new? Thanks