This is what I found out:
There's a thread about Frogger on Polish forum:
Post n.16 refers to the email address of the FroggerNG author.
Post n.26 informs that the answer came from this email (yay!) where the FroggerNG author said he's unable to generate the keyfiles because he has no Amiga hardware anymore.
Maybe this is the good reason to suggest him to use the WinUAE and to generate some more keyfiles. Maybe he can consider to revive the development if asked nicely. It's worth a try.
@stopthego
I know he is still taking registrations. I registered Frogger like March of this year.
I paid through regnet.com. 3-4 days later I got a keyfile.
Really? March? The post number 26 is from June. Hmmm...