This may sound weird, but I bet it will help you out of your situation. I want you to remove your muffler. Once off check the exhaust port and make sure that there is no carbon built up inside it, if there is, clean it out. Now get some hearing protection and start your mower. Does it run up to full speed? If it does then your muffler is most likely carboned up, you may be able to clean it out or you may have to replace the muffler, either way you will be up to full speed again.
It does, I read this as being a 2 stroke lawnmower.
You can still check this, but I doubt that it is your problem as dirtbikes run wide open most of the time. I will opt out and let someone better with dirtbikes help you out.