Not sure about a single cylinder engine, but when I put a new cam/heads/intake on my 350, the proper engine break in procedure was to start the car and bring the rpms to 2000-2500rpm for 20-25 minutes to break in the cam. Then let it cool down. Change the oil and light on the gas for a 500 hundred miles. With a small single cylinder engine, I would start it up and let it run at about 1/2-3/4 throttle for about 20 mins and then let it cool down and change the oil. I would go through a tank or two of gas at no more than 3/4 throttle with a light load. After that the bearings should be worn in and the rings seated so you should be able use it as normal. I've never rebuilt a small engine, so I can't guarantee this is the correct way to break in a little engine.

Good advice but I would work it under light load at full throttle to maximize cooling.