When trying Reaper, I didn't notice Cakewalk Sound Center amongst the Virtual Instruments I could use.  Is there a way of opening it in DAWs other than SONAR?  It'd be good if I could because I did invest in the entire library of sounds for it.

AFAIK, none of the Cakewalk synths are lock to SONAR.
 
Check the scan path. It should run as well as any 32bit synth. If you can do without the 4 buttons on the right-hand side just above the keyboard consider using Rapture Session to play CSS programs. Rapture Session is the 64bit replacement for CSS.
 
 

I think it is under Rapture player or something like that.

I am using it with no problems in Notion 6 and Studio One. It is inside of the Rapture Pro synth.

Yes, Rapture Session and Rapture Pro look for legacy CSS, DP and Rapture programs during install, copying them into their program folder. Both Rapture Session and Rapture Pro have the same limitations when playing CSS and DP programs. Not all features from both synths are 100% replicated.

Thanks all, I didn't realise it was under Rapture. I'll know where to look now! 

To be clear, CSS is listed in Reaper plug-ins if it is in the scan path otherwise Rapture Session/Pro is a better solution if you don't miss the features not replicated by the synths.