If the paint's gone off, well.... it's gone off.
I would be thinking about replacing it rather than trying to resurrect it. Compared to the cost of a new model these days, paint is cheap. It's arguably the cheapest part of the system, but it's the paint job that makes or breaks a project.
It's far easier to buy new paint than it is to strip a paint job gone bad. It's also cheaper than buying a new kit.
However, if you absolutely must try, use the manufacturer's own dedicated thinner, add small amounts at a time and stir well. If you encounter any lumps or strings of paint which won't go away no matter how much you stir, toss it and buy new paint.
If you do successfully "restore" it, test on an old junker first before committing to a "live" build. Also, be prepared to use it all up ASAP, no more than 2 weeks at the outside.