Hi
I have examinned different materials for this goal and most of them worked well. you can use :
1) acetone. first use it with a piece of cloth then use it with a hard tooth brush or better with a brush with very fine metal bristles used for cleaning the tools.
    note if u use toothbrush after an hour the bristles will solve in the acetone so have 2 or 3
2) 2000 thinner again with the same tecnique of acetone
3) chloroform can be used only with metal bristle brush, note it sometimes has a bad effect on some alloys so before using it try it on a very small part of
    your engine crankcase.
4) you can use the following technique only for the aluminum alloy parts of the engine excluding the shaft, bearings, piston and liner
   deassemle the engine and put in boiling water with some detergant in like the powder used in dish washer. take care: NEVER boil the exhaust with other
   parts because the carbon layer inside the muffler detaches the muffler and attaches to other parts and it could never be cleaned. muffler must be first
   completely cleaned from inside with the methods described or even with a soft sandpaper prior to boiling.
   after boiling there might be some remaining stain which could be removed by anti tartar toothpaste and a tooth brush.
5) finally I gained the best results using the intra oral sandblast (air abrasion) used in dentistry, with 50 micron particles of Aluminum oxide. but because of 
   abrasive characteristic of the particles all inner parts of the engine must be sealed to deter sandblast paticles enter inside the engine. after air abrasing
   the engine case you really can not understand the difference between your engine and a new one. this method onlly can be used on outer suface of
   the parts which are not highly polished in the factory.

   Ehsan</p>