First thing to know is what transmitter did the previous owner use. If it was Futaba, then getting his setup info would be very helpful, because the CGY750 can have a lot of gain adjustments made from the radio.
Most important thing is for the Castle...
You will need to:
1 - reverse the throttle channel on your 14SG
2 - set the end points for the Throttle for the castle.
If it was Castle setup with set RPM, then you will need to set throttle curves flat at: 30%, 70% and 100% to use the three different headspeeds.
So of course you will need to connect to the ESC with the Castlelink to see what the settings are.
Now having said all that, in your situation I would...
Review all the existing settings and write them down
Then start everything from scratch so that you learn about the devices and their programming.