Ha-ha good luck to you...
I can only think of two things to do and I'm not sure I should say on here, but...
Option 1. Use them.
You tried to find them and could not, so maybe the chances of you getting a copyright strike for using them is incredibly low if no one can find them. (they are not Pokémon trees or Sonic assets which get flagged up immediately)
If they were on websites and the artist clearly had Trade Marks on them, and asked you not to use them and yet you disregarded their wishes then that is totally wrong and you should not use them. (But you have tried very hard to find them)
I do not know the in's and out's of it but; even if you did get the original artist contacting you, the worst case scenario is a slap on the wrist and you will have to take them out of your game and use something else.
Option 2: Edit them
Open them up in Mario Paint or what ever Photoshop software you use, and change the colour of them a bit, add a few more branches, and just tart them up.
(How much do you need to edit a picture like a tree until it's a different tree? I do not know and it might be worth asking on a forum)
Like I said I don't know the legality of it, but, worst case scenario, you are not going to go to prison for using them :)
But these are just my opinions;
Disclaimer:
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Any way mate, good luck with what ever you do :D