Check these things.
1. Pull the plug out of the head still conected to the wire kick it over and see if you get a spark? If it is not then replace the plug check the key switch,
2. Make sure the carb is getting gas, if its not then check the petcock and fule line.
3. Make sure the air filter is not clogged with anything like a rats nest! Not saying thats the problem but something blocking the air way can mess it up.
4. If its getting gas, and spark, and still not fireing- go to a honda dealer ship and get a honda spark plug trust me it does make a difference, some of the cheap ones at advanced auto are crap, spend the extra 2 bucks and get a good plug.
5. If none of these things work pack it up send it to me and i will take care of it!!!!
6. Just kidding but if you have spark and gas the timming may be out. if someone says the motor was rebuilt, unless they know what they are doing and they checked the timming it wont run even if its out a little.