I looked into replacement GPS units when mine had failed eight months into the warranty. I loathed the thought of packing up my scope and returning it to Celestron.
I eventually fixed my GPS unit myself (replaced $3.00 super-capacitor) but I was considering replacing the GPS module leading up to that.
I balked at the price of the Celestron replacement, I think it was $130, and I thought that they're making a killing on what probably is only worth 10's of dollars nowadays and that there was probably some standard unit that would work in it's place.
The old nexstar GPS 11 schematics and materials I found on the web talked about a motorola unit that uses a small lithium battery for backup and I looked at a lot of potential candidate replacements for the GPS module. But what they're using now is a GPS module made and designed in Taiwan that uses a super-capacitor for backup.
I don't know how compatible the communications protocols are between units. I know there's the standard NMEA, but I don't know for sure if or how they're using that. Then, there's that interface board between the GPS and the rest of the system. What is it doing and how is it affecting the communications? That's about where I lost interest in delving in deeper to the protocols.
I searched and found what I think is the identical unit from a Taiwan supplier but they're sold only to OEM's and cannot be bought singly. I got a quote for a sample order with a minimum quantity of five for $30 each. I would have bought one to try, but not five.
As an aside, when I was probing around on the serial bus for GPS communications between the GPS interface board, the GPS unit, and the rest of the system, I disconnected one of the serial connectors on the GPS interface board (I wanted to see which way the data was flowing). After that I got boot 008 messages from the hand controller and could not recover even after reconnecting the interface board!
It didn't surprise me that things stopped working when the interface board was disconnected, but it did surprise me that the hand controller didn't recover when communications was re-established!!! I thought I had toasted something.
I eventually recovered by re-flashing firmware on the HC but that's another story. Then I replaced the super capacitor and my GPS has worked well ever since.
I applaud your goal of learning more about how this works. Please keep us updated!