Correspondence suggests that a blocked exhaust is one of the most common causes of a chronic loss of power. Over the years, a black deposit varying in consistency from tarry to flaky builds up inside the exhaust pipe. This narrows the internal diameter of the pipe, restricting engine output. Most of the deposit usually builds up in the first fifteen to twenty centimetres of the pipe, so cleaning it is not difficult, although it is messy. Sometimes, however, the narrowing can extend further. The Webmaster has a hot exhaust pipe running straight back from the exhaust gas collector, and has found his exhaust choked for over a metre from the engine.
You'll probably have to remove the exhaust, just to gain access. The thumbnails above show the removal of one of the several exhaust configurations that you may find. Even if yours is different, the principle is the same.
You may be able to scrape the inside clean with just a big screwdriver. However, best results are achieved by burning off the deposits. You can use an ordinary gas blowtorch or, as in the illustrations below, an oxyacetylene system, to get things really hot. Whatever you do, BE CAREFUL. Wear appropriate protective clothing, conduct the operation outdoors if you can, and always have fire-fighting equipment to hand.