I tried some grease/thick oil thinking the same thing - though I know the stuff I used isn't water proof. Didn't help a lick. I suppose there could have been some kind of a flex washer but how it would have mounted on the piston I wouldn't know (doesn't seem to me it would have had one).
I was hoping the whistle top that broke off was an insert into the body, once I got it under the magnifying glass that isn't the case. I think my only hope is another (not correct) whistle or another Empire donor whistle. I will be so bummed if I have to get a "replacement that fits" as it would really mess up the original look of the engine. I just might stick the whistle top on just so it "looks" right - just have to figure out a way to make it stay on.