About 6 yrs ago my Ehiem started leaking at the output 'nub' at the top and I had to take down the entire system and have not run a tank since. Once they leak from that location, there's nothing you can do about it except have a replacement ready to go and who does that.
Fast forward to today, I have designed a 5 gallon bucket canister with an external pump& bulkheads. It's not operational yet but my prototype is fully operational. I will never use an Ehiem again and to answer your question, the parts you need to have on hand is a complete replacement other than that cheap piece of plastic garbage.