I've done quite a lot of expert witness work, although this would be well outside of my personal expertise - and I'm the other side of the world.
The courts will likely accept anybody who is sufficiently experienced in the subject matter, capable of writing good prose (there'll be a lot of report writing involved), and completely independent of any party in the case.
I'd look to a large local R22 operator/maintenance organisation and see if you can buy some time from their Chief Engineer or Quality Manager. Tell them to find out what the lawyers in the case are charging per hour and to charge about the same.
G