One of the best ways to determine if a part fits your Kadett is the Opel Part Number - especially for NOS parts on EBay that are still in their packets or a part number is quoted by the seller.
To know if these parts fit your Kadett A you need an Opel Parts Book which Lists them. The one you need looks like the picture below and the other pic shows a sample page.
For example you can see under "PUMP, FUEL" that (19) 64-67 Kadetts all use the same fuel pump, Opel Part Number 816021. By looking at the later Opel and GT parts book that same pump is used on all 1.1 litre Kadett motors from (19)64-71 and 1.1 litre GT motors from (19)69-70 so that gives you a wider range of options for a fuel pump.
From time to time these Opel Parts Books turn up on EBay for quite modest sums of money. Infact there are some in the Buy Now Section of EBay now.
Here:
eBay Motors: Opel 1966, 1967 & 1967 1/2 Kadett Parts Catalog Manual (item 320087355917 end time Apr-28-07 12:15:12 PDT)
AND Here:
eBay: 1968 1969 1970 1971 1973 OPEL KADETT GT PARTS BOOK 72 (item 6605495409 end time May-09-07 11:47:47 PDT)
And look at this Workshop Manual:
eBay Motors: COMPLETE 1962 TO 1970 OPEL KADETT WORKSHOP MANUAL (item 8076143017 end time May-11-07 21:19:08 PDT)