What oil ratio are you running? If you're following the kit instructions and using a 16:1 mix it's a good bet your muffler is getting oil fouled.
Try cleaning the exhaust system and dropping down to no more than a 24:1 fuel to oil ratio.
Let us know what you find. There are other things we can look at that could cause your loss of power.
Whatever, do not run your engine without the exhaust/muffler. You'll be replacing the engine very soon if you do, and your neighbors will be forming a lynch party.
Tom