Question

Is it plagiarism/copying to do a science fair project that is similar to one that has been done already, if you modify certain aspects and you do not know the results?

Answer 1

It is not as plagiarism as it is different project. You are even allowed to repeat past experiments, although you should have a category in your presentation of "background research". Science fair projects are about obtaining research on your own through you personal experiments. It sounds acceptable, but it would be advisable to double check it with your teacher or supervisor.

Answer 2

If you cite your sources correctly and make sure to credit everything I'm p sure it's cool. You might lose points on originality but it depends on the teacher