Interestingly, I've just gone down the rabbit hole of attempting this mod myself, or well, at least having the BGA part done by someone with more BGA soldering skills than me. I've pulled the trigger on just buying
this card from eBay (note to mods, I have no affiliation with this seller beyond buying some of their wares at full price with my own money). I did also buy an extra drive sled and replacement CD bezel, but I ended up paying more for these parts than I did for my O2. Oh well, the things we do for our hobbies! He's still got one left, so it might be an option. It's certainly cheaper than anywhere else I've found.
Please do let me know if you know a place willing to do the soldering work, given I know you're in the Netherlands too, as I've seen the Marktplaats wanted ad you put up for this as well. Same for the Xilinx PROM, while I'm currently looking into the alternatives, if you can get your hands on either the blank ICs and a programmer, or pre-programmed ICs, I'd be interested in getting one too.
There's some info on potential replacements in the smouldering remains of Nekochan, specifically kubatyszko's post at the end of this thread:
It mentions there's a 20-pin Xilinx SOIC chip that's compatible with the 8-pin Xilinx SOIC used on the O2 cards, which in turn has a modern compatible replacement that's JTAG programmable and even re-programmable. If I can't find some of the 8-pin chips pre-programmed, I'll end up getting the bits necessary to get the modern 20-pin chip working. There's a cheap FPGA kit out there that has one of these chips on it, that with a cheap FPGA programmer (both available from Chinese sources) could be programmed with the necessary code for the O2 board. In turn, that 20-pin SOIC chip would be soldered to a 20-pin SOIC breakout board, with wires going from the appropriate pins from the 20-pin SOIC, to the solder pads for the 8-pin SOIC on the O2 board. Not particularly pretty, but that should work, though I still want to verify this.