Best upgrade for cd player?

Depending on the cartridge, a VPI classic can out-dynamic, out-detail and out-nuance even a really good CD.   

But I think the OP might be looking for a DAC.  Use the existing CD player and coax connect into the DAC.   Lots of used DACs here, on ebay and elsewhere. Some of those are probably reasonably satisfying.  But that they are for sale doesn't speak very highly of them since it seems their owners weren't satisfied with their performance and wanted something else.  

Get a good DAC.  That’ll give you more of an improvement than pumping more $ into an Oppo that’s out of business — good luck when you have a problem.  What’s your budget?

I can tell you that a bettrer cord and interconnects can make a difference as well as a dedicated conditioner that separates the digital from the analog equipment as well as a dedicated line to plug all your gear into.

Get a transport and send it to a tubed dac....

ptss

Purchase a dedicated, reference, CD player.

Happy Listening!