Methylene Cloride, Trichloroethylene, Xylene, and Ethyl Bensene
A little MEK, xylene or toluene should do it. Acetone probably evaporates too quickly.
You could probably use generic "lacquer thinner"
The first suggestion at least mentioned one of the ingredients. How about a little rocket fuel to thin it? Nitroglycerine?
The chances of making something toxic or dangerous is a real possibility. If you can't get what is listed on the can I wouldn't do anything.
Methylene Chloride is in some paint strippers, but there may be other chemicals in it. Xylene is a paint thinner and you can get it from the local paint store. A very small amount should be used. When it is thinned it might lose sealing properties.