Offering Authorship Prospects To Professors For MSc Thesis
Since you are tentative about whether this would be acceptable, I suggest that you put that idea off for a while, at least. Talk to your advisor about the issue. It might be that co-authorship of any kind might not get you to your degree. You need to find out about that. 

But the best option might be if you could work with someone with more math experience, even a professor, who could teach you the techniques you need. At a minimum they could offer advice about how the original paper was done so that you can reproduce the results. They might just be able to point you to a source for learning the needed technique. 

This might be considered different than helping you with your own paper and might not result in co-authorship, depending on the questions you ask and the help you get. An acknowledgement would be appropriate, of course. But if co-authorship is ok at your institution, then a local one might be better than one with the original authors.