If you can get access to a local airfield or similar, there is an aircraft product that's called alubright. It's similar to a mild caustic solution that cleans the dirt off, whilst giving the surface a bright clean finish.
You apply it with a brush, then wash off after 10-15 mins
Make sure the heads and barrels are de-greased, Gunk or Jizer, then cover the timing chest primary etc with some old WET toweling then spray and bush on Cillit Bang, leave for 15 mins then hose off, careful because anything not protected with wet towels or cloths will be stained and will need re-polishing, and you know what a sod that can be!!!