Thank you for the workaround. For those that may run into this issue I was able to change the long to a double:
Thank you!
JScriptEngine engine = new JScriptEngine();
engine.Script.bigNumber = (double)1400000000000;
var result = engine.Evaluate( "'BigNumber: ' + bigNumber" );
Console.WriteLine( "Result: " + result.ToString() );
The number was output correctly.Thank you!