You can use modules and use your SY-77 as a controller. What module depends on how much you are prepared to spend. Roland has the Roland Integra
Yamaha has Motif XS
Of course you would need to change patches from the module and not the SY-77. While these modules aren't cheap they are still much cheaper than the keyboards that have the same sound engines.


Though the cheaper alternative is to get something like the Roland RD4 Digital Piano and MIDI it to your SY-77 for synth sounds but of course that does mean using 2 keyboards on stage. But it does also provides organ and clavinet sounds. [video:youtube]https://youtu.be/ybs0ukrluB0[/video]