I have had great success using a 2 step process. Step 1 is to use #32 glass beads to remove crud and soften-up the alloy. Step 2 is to submerge the part in a vibrating tumbler with various sizes of spherical ceramic media, then apply enough power to light 10 houses for a week (4 hour emersion). The parts come out bright, completely peened, which provides superior dirt resistance, increases the engine's weep resistance and adds a modicum of strength to the parts similar to shot peening connecting rods. The down side is that you must close off or proof all oil galleries; I use high pressure dish soap/water; with the Commando the oil galleries are easily proofed. I leave the to-be-ditched main bearing outer races in place, and plug the cam bearing with a properly fitted wooden dowel, unless I plan to replace the cam bushings, in which case I don't bother to mask them in any way. Any threaded holes will need to be cleaned or occupied by a threaded fastener (barrel studs come to mind).
If you Google "metal passivation" , which is a sister process to ceramic tumbling, you should find a vendor close by.
I have seen a third option that I want to try, demonstrated to me by a highly regarded East Coast Triumph expert, that utilizes a product called, "Pre Aluminum Welding Cleaner" (which is an acid with a strength somewhere between muriatic and phosphoric, I suspect); you'll need a very well ventilated place, our doors is best, a water supply, and protective gear for skin and respiration. The end results I have seen are pretty good; a bit darker then when the alloy was new (as mentioned earlier in this thread). I do have a concern about this process increasing porosity and making the alloy more susceptible to taking on dirt, grease, leaks of any kind.
rd ... sting.html
