My John Deere loses power.

Hi everyone. I have a John Deere X165, it is a V twin, with a Briggs and Stratton engine. Until recently it has been a beast of machine and has made short work of mowing my lawn. However in the last couple of weeks it has started to lose power. I initially knew something was wrong as it didn't sound right when I started it up, it then was very sluggish and when the blades were engaged was barely able to cut the grass. I did the usual checks before getting in touch with the local dealer, who were very helpful. I have not taken it to them as yet, preferring to have a go myself, they will charge £60 an hour or about $80. It has only done 49 hours, but is out of warranty.
I have carried out the following: Changed the fuel filter. Changed the plugs. Put new fuel in. Taken the carburetor off and found the 2 small jets had worked lose and were in the bottom of the bowl. I replaced these thinking this was the problem.
It still has the same problem though. Can anybody help please? Has anyone else had this problem? I am very disappointed to say the least. Previous to this I had a Husquarvana for 17 years without any problems!!
Thanks Wayne

May need to get an inline spark tester, and check for spark when the engine is running.

Did you take it back for it's initial in warranty service ?
Have you ever checked the valve lash ?

Sounds like you have lost one cylinder.
Is there a difference in the temperature one side to the other, particularly at the exhaust pipes.