ValvesFor the valves I use a brass or copper wire brush as it is important to not score the valves or valve seats. I usually clean them in situ. If there is hard build up which the brush will not remove use a flat bladed wood chisel to scrape it away.
PistonsFor the pistons and inside of the cylinder head I start with a flat bladed wood chisel about 1" wide and scrape away the carbon buildup from all the surfaces. After scraping I go back to the soft brush and finish the job. When done feel for burrs or nicks and remedy them with 600 grit sand paper.
Exhaust Manifold & PipesNext onto the exhaust manifold and pipes, the build up in here is usually very soft, sticky and oily. I scrape out the worst of it using whatever will fit in the holes, usually sticks of wood are good for this. When just about clean I'll clamp the unit in the bench vice and start heating the manifold or pipe until it starts to smoke. Then I'll try and ignite the contents using the blow torch. Make sure the pipe is well clamped, they make really good rocket engines and can fly for quite some distance (been there done that) and while it is funny to watch it is very dangerous. I once blew the baffle out of a motorcycle exhaust I was cleaning right through the door of my junk car.
Intake ManifoldGenerally there will not be any carbon buildup in the intake manifold unless the engine has burnt dirty fuel or there has been some ignition in the intake. One of my engines, the Southern Cross 'P' is a kerosene burner, and the kerosene is pre-heated before carburetion by passing it through a spiral tube within the exhaust manifold. In this engine the spiral was so choked with carbon that almost no fuel was getting through. One of my first dirty jobs in the restoration of this engine was disassembling and cleaning this. I heated the tube then dropped it into degreaser for half an hour or so.