Addendum: This happened earlier. I only got to write it down around Feb. 2019.
It is clear that working with a C64 with only a dataset or real disk drive not up to bar nowadays. Therefore I ordered a Pi1541 floppy emulator. A guy from Sweden has a quite cheap kit which even contains an OLED display. There are many variants of the circuit on the internet. I think the one I ordered is a more mature version with two sockets to loop through to another floppy.
By the way, the guy kirsti_73 has cool stuff in his eBay store. If the Pi1541 works out, I might order the kit for the Final Cartridge III.
I’m curious when the part will arrive. I also have to build a serial cable for the C64.
Since he had a Diagnostic Cartridge rev. 586220 in his store, I started researching about it. The cartridge he sells doesn’t do much on its own. You need a set of cables to connect to all ports. There are several versions floating around on the internet. Especially quite cheap ones on eBay, which only have connectors that are not connected to each other.
On the page of Peter Schepers you can see a better version. There is also a schematic: https://ist.uwaterloo.ca/~schepers/diagnostic.html
This site World of Jani has a great overview of the diagnostic cartridges plus rom images and documentation. The picture from the Commodore manual of the 586220 looks something like the circuit from Peter Schepers:
I also found an offer on eBay:
I found out afterwards that you can even see this on a picture in the eBay listing I bought my C64 from:
Somehow it would be cool to build something like that myself. I think I would have to layout a real board. Maybe it would also work with a breadboard.
PS: I should read this thread.