How update Android on G1 manually?

I have a G1 in orginal state using firmware 1.0.
I'd like to update the OS with a later version.
How do I do this? I had expected this would be well documented but can't find anything useful. Perhaps I'm not searching for the correct terminology.

I'm in India using a PAYG SIM card so don't want to update over the air, even if that were supported, but to download an image or build to my laptop and update my phone from there.

Where do I d/l official Google releases and how do I install them?
Presumably the latest releases require better h/w than the G1 has.
What's the most recent release that will run on a G1?
I've learnt there are ROMs that are images that add improvements to the official releases.
Which of these would be good for the G1 and where do I find them and how do I install them?

Assuming all the above succeeds, I'd like to be able to play with the Android code. Where do I find the code so I can rebuild the OS from source code after adding my own code.
For now, if I could just add a popup 'Hello, World' after the phone has booted I'd be estatic.

This is obviously rather broad a request. If there are detailed websites out there that provide manuals and code and tutorials I can't find them but assume they must exist and will be very grateful for links.