I am very interested in this and I personally would definitely go at it, but I am currently still lacking the technical acu(wo)men 
It’s going to be challenging without a doubt.
Even with the source code, building the FAW custom firmware we have here took months of working almost every day.
That was with a primary dev who was very skilled and me working on it.
So, be prepared for many, many hours of debugging issues with the very custom modem files and screen resolutions and related overlays.
Certainly not a job for the feint hearted and you don’t have the source to work with.
Everyone gets excited about this but when it comes to the crunch, not a great job at all.
It’s definitely worth mentioning that if you did this, you would need to use an AOSP base that has never been in contact with the GMS ecosystem.
Trying to reverse engineer a firmware that has GMS in the mix will be even more time consuming and end with battery drain issues.
Just throwing in my 2 cents…
Good luck 
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