Hello Jasan!

Mathematics is a basic requirement for Mechanical engineering, be it in India or abroad. So, it will be a basic requirement for you during your application to various universities abroad.

I will suggest taking the Board Examination for mathematics only through NIOS. This way you'll be able to add mathematics to your mark sheet. You do not need to PCM all again, appear only maths as an additional subject. This is because you already have a PCB mark sheet, and after appearing NIOS boards for maths, the marks will add to your mark sheet.

For studying in universities abroad, you have to qualify for SAT/ACT and language proficiency exams like IELTS/TOEFL. Go through the articles below to get more information about these exams in detail:-

All about the SAT:- https://www.google.com/amp/s/studyabroad.careers360.com/articles/what-is-sat/amp

All about TOEFL:- https://www.google.com/amp/s/studyabroad.careers360.com/articles/toefl/amp

All the best!