Modifying Kundal's script slightly, JScript's encodeURIComponent covers a few more cases ( , / ? : @ & = + $ # although not all are possible in filenames to start with of course).
(The "replace" line is because it needs to double the % characters before running the command.)
function OnClick(data) {
data.func.command.ClearFiles();
var objEnum = new Enumerator(data.func.sourcetab.selected);
while (!objEnum.atEnd()) {
var item = objEnum.item().name_stem; objEnum.moveNext();
var encoded = encodeURIComponent(item);
var search = encoded.replace(/%/g,"%%");
data.func.command.RunCommand("http://www.google.com/search?q=" + search);
}
}