I thought that 1MB of RAM was/is a little low. Therefore I searched the internet for an affordable 4MB (or bigger) RAM card. I found one made by GGLabs and they also offer(ed) bare PCBs.
I ordered two bare PCBs and went searching the internet for the 74xxx logic chips, together with the 20-pin SIP RAM modules which are supported for this card. I found these through AliExpress.
Unfortunately I wasn't able to find the resistor arrays which GGLabs originally used, but using (small) generic resistors also worked.
The RAM card is correctly identified. I did a burn-in test using the Sirius image (from the old sector16.com site) for at least 48 hours and everything was/is still fine.
Right now I have 2 4MB RAM cards, one for each IIgs.