i'll try help the case here.
You tried to contact Kamil and didn't manage to. I don't think you will be able to. Last i talked to him 6 month ago, he said he was busy with his life and projects, and couldn't afford time for the mod anymore. Sad situation as he did great work, but good for him and his life!
To me, and that's just my opinion, as long as you state it is Kamil initial work, and offer to download only the file you changed (or rather, created...), and that you don't ask compensation for it. I think it's ok.
It's a situation where the mod is kinda dead because
A/. the sound doesn't work since kunos patch 1-something
B/. there is very little chance the owner will update it one day...
I see what you offer as an "update" or rather an "extension"
If you made the skin yourself of the actual 620r, that would be even better.
Good question about the sound, did you make it?
since it's been mentioned, i might do a 620r later down the line (-why do a r500 when you can make the ultimate...) But first i need to have a final version of the current version