DIY searches turn up very few references that are useful to the DIY enthusiast or hobbyist. The days of do it yourself kits are pretty much a thing of the past. Anything that is remotely close to something that may be useful is usually quite dated. Most often you would turn up a page that may only look promising. Make sure you look for a date of publication or some hint as to when the information was published. You’ll find these to be as old as the early nineties or the beginning of the current millennium. That kind of dated information would not be useful as the parts used in the schematics would not be available on the market now. I am not sure if the kit business existed in GPS receivers at all. Perhaps it did in the very early part of this developing technology but remember that GPS has been around for four decades now, easily. It has gotten to be so sophisticated that there is not much point in trying to develop your own. The kind of development and testing tools you would need is only affordable for the deep-pockets of the development labs at products companies. The circuit board you would need to make would be quite sophisticated and you would need suitable skills to develop mixed signal boards. Once again, these are in the realm of product company development labs, not the hobbyists. Here is one reference which could have been useful but it was published way back in 2004.