Well if you want to pay to have it done you will need to disassemble it. If you went that route you could get bead blasting or even aqua blasting. But that you will $$$ threw the nose.
If your a DIY type of guy, and don't want to disassemble the motor, i would seriously look into Soda Blasting. You can get a unit from Harbor Freight for on the cheap, well relative to the cost of the other options. All you really need is a decent air compressor. There is a ton of information about the idiosyncrasies of soda blasting on this forum, just put it in the search box. I used it on one my engines and it turned out great.
If not borrow a steam cleaner/ pressure washer, and hose her down with some degreaser. No where even close to a soda blast but cleans it up. I just did that to the one i am working on, but its a gravel /dirt bike and i am not looking for a show quality motor, so it fit the bill over soda blasting. I also used a pressure steamer i use to clean off tractors on the farm so it was a "serious business" type of one. I could send you pictures i suppose if you really wanted.
