Odds are you do have documentation. Get all the emails, receipts, etc. you can find, the code (hopefully you have logs, etc. of work in source control, although I'm not sure if they'd know what to do with that in court or if would be admissible as evidence), and talk to people who could testify on your behalf.
The other option is to ditch it all, but it sounds like you are invested enough where you would rather make a lifetime enemy than give it all up.
I'd think you should get a good lawyer asap.
Odds are you do have documentation. Get all the emails, receipts, etc. you can find, the code (hopefully you have logs, etc. of work in source control, although I'm not sure if they'd know what to do with that in court or if would be admissible as evidence), and talk to people who could testify on your behalf.
The other option is to ditch it all, but it sounds like you are invested enough where you would rather make a lifetime enemy than give it all up.