Hi kevbar. We will need more information before we can be of much help I think. Can you tell us the model of brushcutter, about how many hours it has run, and post a picture or two please?
Generically, there are two things that will need investigation right at the beginning, before considering more subtle possibilities. The first is how long you have been using your current spark plug. As a simple rule of thumb, the spark plug used in a well-tuned two stroke can last about 15 hours before engine power falls off substantially, but only if the engine is mostly running warm and under load. Otherwise, it may last considerably less than 15 hours. Power loss in the vicinity of 40% is to be expected to arise fairly soon after the 15 hour limit. After that, power will continue to decrease.
The second problem that arises happens over a longer period, perhaps as much as 200 hours' use, if the fuel always contains the right amount of the right oil, but can happen very much sooner if the oil supply is a bit skimpy. That problem is loss of cylinder compression. It is usually mainly due to piston ring wear.
If you give us some more detail, we can talk about how to investigate to find the specific problem.
