Morris,
I have been soaking my spark plugs in a 50/50 mix of household ammonia and citric acid. (I had noticed that the small bottle of cleaner supplied with the ColourTune is ammonium citrate) I'm trying to optimize my carb mixture by reading the plugs, so I need to clean them all the time.
The ammonia (nasty stuff!), of course, you can buy at the grocery store. I already had some citric acid (bought on Ebay to clean my homebrewing kegs of "beerstone"). It is a solid and not too dangerous, though if you have children/pets you have to be careful, of course. It is a white powder.
I noticed the mixture also seems to slowly dissolves the metal of the spark plug (or at the least discolor it) so I wouldn't soak your precious parts in it overnight, for example.
Good luck with your engine work!
Cheers!
PS. I assume you are talking about your Midget, which is all cast iron, right.