Hi there!
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You can use the 4.20 release, change the engine release associated with the plugin and compile it for 4.21. There were no changes from 4.20 to 4.21. Note that any fixes in this project is only made at the latest release, so you might want to have the latest engine release installed and check GitHub and copy those changes to the release you are using, those related to C++ at least, since it won’t work copying an asset from a newer release on top of older release.
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I think it is possible, since the wake is nothing more than several planes dropped at the water surface while the boat pass and those planes are destroyed after a certain time. Those planes receive a translucent material, so it would be in that material that the normal should be added. You can do it yourself if you wish.
Cheers!