Its a long process that is probably done a bit at a time or something that can be done over a period of time.
Scotchbrite, stiff plastic brushes, and a degreasant are the key - plus a light oil (gt85, wd40, etc)you can spray on afterwards to protect the finish you have achieved.
Acids must be used with care - do not leave them on for long and test a small area first.
Media blasting should only be done if you are stripping you engine to individual componenets and then you should ensure that everything is clear of the media before you rebuild - this is difficult. However you can blast individual items - front and top cover and top ends if you have them stripped down. But be prepared for a 'two tone' engine and gearbox.
The point is, is that there are no 'silver bullets'.
Please do not be tempted to polish your engine......you will enter a 'world of pain' and the finished product will not be to everyone's taste and is not the finish that BMW meant to have for thier bikes. You can tell that I am not a fan of highly polished bikes.
"Chrome won't bring you home"
Steve H