I appreciate the OP's question, but it seems to me that
One can rewrite their books in modern language and notation or guide others to learn it too but I never believed this was the significant part of a mathematician work; which would be the creation of original mathematics
is a widely-held fallacy. Perhaps 90% of the work of mathematicians is not brilliantly original creation, but the challenge of digesting and reworking over the cumulative insights and re-presenting them in a useful way for others (this generation and the next). This is a huge task, and it's often a very satisfyingly creative task which will well reward one with new insights, understanding of consequences of the major insights, and understanding their underpinnings.
And part of this is: teach and write well. Good exposition is perhaps the most important contribution that "lesser mortals" can make (and perhaps pourour collective livelihood depends on it -- Rota has some perceptive things to say about that in his Indiscrete Thoughts). You can get started on this even as an undergraduate: once you have understood something, share it with others. Give a good talk in your local math club. Pay it forward!