Ok small update
I finally got to snoop the 250 being used as external receiver to the 150 for guidance.
What i know for sure is the the Arduino nano is too slow to pull this off, after the initial handshake at 9600 the units ramp up too 115200baud. This is outside of the limits if the Nanos 57600baud max.
This is also wild cause you can only manually set the 150 to 38400baud.
Also after recording the traffic in both directions almost none of the messages used in the existing converter are required or used.
My hopefulness that it used GGA at the agreed on speed was wrong. It seems to send all the satellite data in raw format to the 150. How much is really needed to function is to be discovered.
There is also a definite call and reply to the messages. I am unsure if flooding the guidance system with the needed messages out of sync will suffice. The converter will most likely have to be smarter than just a one way dump.
It looks as though the workflow will have to sort data from the NMEA, put it in registers in tsip in converted form, ready to be sent for when it is called upon by guidance.
Also you would have to never update your guidance firmware cause one new tsip command change could mess it up.
This still seems possible and a fun challenge, but its making AgOpenGps, the original 150 and Teejet look so much better in price and value and longevity.