What I just did - I opened up ebay, wrote "volkswagen brake master cylinder reservoir seals" and ended up with this
https://www.ebay.com/sch/i.html?_fro...seals&_sacat=0 . I wrote volkswagen, because both used Girling stuff, but VW is internationally more popular.
As for your engine running problems - it's hard to point at something - I usually just go over all the system checking if all is fine. I've created KE-jetronic diagnostic thread in this forum, btw.