Want my OPPO UDP-205 to sound as good as my Sony SCD-777ES

It would be pretty difficult to do that and get it RIGHT.
What is it about the Oppo sound difference that you want to change?
Are using the "audio outputs"? Or as a transport to a separate DAC?
If using the "audio outputs" line outs;
You could look into Swapping the audio output coupling caps to Mundorf EVO aluminum/oil caps.
I wouldn't change anything on the Oppo.
Wait to see if others reply; who know WAY more than me about good mods.

"Sony players that still sound great are starting to fail." The thing that is going to fail is the LASER and possibly the sled/drive mechanism. I'd contact Sony to see if these parts are available and buy two or three of them (each should be good for about ten years). BTW I'm a proud Oppo 205 owner and have no problem with the way it sounds.

I’d be surprised if Sony stocks the laser you need. When my Sony SCD-777 failed, I bought a replacement on eBay and had a friend install it. No problem! Be prepared, as some lasers are hard to find and expensive. My Oppo 105D sounds as good to me as the Sony.

Sell the Oppo for a premium and use the money to buy a dedicated CD/SACD player.I have no complaints with my Oppo. It sounds better than the dedicated SACD player I once owned. Good luck.

I bought a 205 and it’s a fine player, but I prefer my Cambridge 851C. If I didn’t already the Cambridge I’d probably be fine with the Oppo. The Oppo is stock but I might throw in an OppoMod PS and then a fuse but that’ll be it. My Cambridge does has two Synegestic Black fuses and that did make a nice improvement to a fine player to begin with. The Cambridge does not play video or SACD and that’s fine. The DAC in the Cambridge is very good and I’m running my Bluesound Vault 2 through it. Tried both the Cambridge and Oppo with the same WyWires balanced cable and the Cambridge is just better, used same Nordost Vishnu power cord so the comparison was fair.

Those that have done the ModWright upgrades love it and I don’t doubt they are very good but I’d rather spend that kind of money on a separate DAC personally. I’m on my second MW preamp and know Dan makes great gear.

Guess I’d suggest the OppoMod PS, then you can look at a fuse upgrade and buy a nice PC to finish it out. Nordost PC are fast and very revealing on digital without adding an edge... at least that’s my opinion.

I just bought a Supra LoRad Mains Distribution Block that I’m very impressed with. Using it on my digital equipment only and too soon to make a endorsement on it but out of the box it’s stupid good for what little it cost. If you’re just plugging your Oppo into the wall maybe look into this.

https://www.madisoundspeakerstore.com/power-cables/supra-mains-power-distribution-block-md06-us/

I would take mine (if I were lucky enough to have one) straight to Ric Schultz at EVS (Electronic Visionary Systems, Calif.). He's been modding amps and digital for more than 30 yrs now I believe and I've been dealing with him for many years. He's really easy to talk to by phone about any subject and he tells me his modded 205 is easily his best yet for modding digital. I have his modded 103 and I love it and Ric says the modded 205 leaves it in the dust. I imagine you might need to prepare yourself for 'much better' than the Sony, as opposed "to as good as".

Like I say, the only reason I haven't done this myself is because I don't have a 205. If you end up wanting to sell yours, we might should talk!