You don't have to worry much about your 12th grade marks now. The main problem is not getting the college to enroll yourself because there are many colleges now throughout India.

The real problem will be when you search for the job as many of the companies will have the eligibility criteria as 60% above in 10th, 12th and UG/PG degree.

Still there are many small scale companies which enroll students with only the UG/PG degree marks and won't be considering the 10th/12th marks.

Repeating of 12th grade will have no problem in getting you the admission in colleges because 1 year gap will not be an issue at all.

So, if you feel that you can repeat one more year then you can actually work hard and prepare more better for your 12th exam to score more than 80%-90% but if you feel that it's not possible to repeat one more year then make sure that you put your hard work and do the best in your college which of-course get you a good job and career for your future,

Note:

You will definitely get admission in bachelor of engineering college dont be so confused adked if you have any other query