Schaublin 125-CNC retrofit.

They offer this widget to support resolvers.
store.granitedevices.com/argonaccessories/13-gdares.html

Hm. But how are you going to connect your synchros to that?

[edit]
Maybe I'm barking up the wrong tree but your synchro seems to have 3phases at 120° and a "standard" resolver has 2 phases at 90°

Since these resolvers are reported to be the wrong style to integrate with these Granite drives, I guess that is a dead end.
Mark

Not glad about it, but welcome to the club !
I have a Mazak that has something that i can only describe as "reversed resolvers" that i have to replace with encoders, and since i gave the small lathe to a "friend" i have to go to another frined to make the new shafts.
Regards,
Tom, the "not helping guy"

Adding encoders to the existing motors is also an option, I need to work through.

What resolution do you need for the encoders?
There is one converter with an nicrosontroller in the wiki:

wiki.linuxcnc.org/cgi-bin/wiki.pl?ResolverToQuadratureConverter

With this you could move to a simple retrofit with keeping the drivers and just switch the control. Cause everting look really nice and neatly organised.