They won't. While it is unknown if there is intelaigent alien life it is known that none is anywhere close. When distances are measured in tens of lightyears for close thing they can't get here and back in time to make a difference. Most distances are measured something much larger. Just the milkyway is 25 thousand lightyears to cross.
note that are farthest detectable radio waves have gone maybe 100 lightyears: odds are your are dead before they know we are here.
No, but most replacement parts are semi universal parts that can be adjusted to fit the vast majority. However you need to know what toilet you have as each almost universal part doesn't fit something another one will. Odds are you have a common toilet that any universal part fits - but there are toilets that are not compatible ith anything.
before doing anything though should you replace the whole thing is a question worth asking. do you prefer a round or engladed bowl; what about the height; maybe you want one without stains; or a low water use version that works? It wouldn't cost much more to replace the whole thing with one that fits you better.