If you've got a well-ventilated work area (like outside, maybe?) and like the aroma of solvents, most local Whatzit Stores carry cans of brake cleaner for about $2. A few cans of that and a collection of *SOFT* wire brushes will do utterly amazing things to accumilated crud, oil stains and aluminum oxides.
The small round-type *SOFT* wire brushes usually found hanging around Safey Klean (tm) parts washers and Sears Hardware stores do a nice job 'tween the fins. A Dremel tool or better yet a die grinder with an assortment of round *SOFT* wire wheels will get you just about anywhere on a motor you want to go, and do it *much* quicker than by hand. A die grinder (electric or pneumatic) is the "Tool Time" version of a Dremel - *More Power! woofwoofwoof* Extra points and less headaches for a light touch. And get the best full-span saftey glasses or goggles you can find and wear them - you're about to spin a wire wheel up past 10,000 rpm. The word for today is *pincushion*.
The reason I'm hyping *SOFT* brushes is that it's quite possible to put leave swirl-marks and even erode the aluminum surface if you get a bit froggy with it. A soft brush protects your parts from the leering schlepp with a power tool in his hands and a burnished-metal dream in his eyes - in other words, you. Go easy. Be patient - much the same as those self-ed books on how to stick your girlfriend to the ceiling, there's a shine in that metal, you just have to bring it out.
Brake cleaner evaporates real quick, so there's a learning curve to how much to apply and how wet to get an area. (no, I'm not talking about your gal-pal here - get your mind back on the metal...) It's possible to do a whole motor with 2 cans, but for a first-timer, add a 1-2 can *Newbie Factor*.
There's scads of grits for Scotchbrite, and the finer ones are okay for aluminum, but personally, I never use the stuff. And it's *DEADLY* to chrome in any form. Anyone found using Scotchbrite on chrome should be permanently demoted to owning nothing finer than a Hoctuie (yes, that's a real (Chinese) brand - pronounced HOCK - TWO - EEE) 50cc scooter.
Beware - cleaning aluminum exposes you to a much more invasive disease - the dreaded *Polishing Virus*. It's possible to buff aluminum to a point that even close lookers will swear it's chrome. I know, I've been trying to shake the bug for nigh on 20 years. But that's another long, long... well, maybe several posts.
For the curious, there's a few shots of my earlier bouts with the affliction in the KZ1000 gallery...