Well, you replace that card with a JetDirect MIO card that has an
Ethernet port. The JetDirect J2552 has ~both~ Ethernet and Localtalk
connectors and is available on eBay right now.
Asante used to make an "Asantrint" device to connect LocalTalk
printers to Ethernet networks as well as an "Asantalk" device to
bridge Ethernet and LocalTalk networks. I think Farallon also made such
a bridge. You might be able to find one of these at one of the used
Apple also made a software Ethernet to LocalTalk bridge that might run
on the PowerCenter:
At some point, you will have to abandon old hardware if you want to use
new hardware or software. IMHO, the easiest thing for you to abandon now
is LocalTalk, which Apple dumped four years ago.