Until the new key fob arrives I assembled the receiver and managed to get the transmitter board inside the old remote.
I've repurposed the existing button and the led to make the remote work.
The original setup uses 2 CR2032 lithium bateries. As I only need one, I removed the other and tighten up the contacts and found enough room to fit the Nrf24LE1 board! Yes!
It's a (very) tight fit but it holds :)
I've made a simple test with the receiver and it works. Now I have to connect it to the alarm. It's a more sensitive operation as I use the car almost every day and can't mess this up or the car wont start :).
Added two connector ports, one for normal use and one for programming in case I want to make a firmware upgrade :)