Hello. You will not be eligible if you give only some of the subjects for improvement. You need to appear in all the subjects of your 10+2 in order to be eligible for BITSAT 2019. BITSAT eligibility criteria mentions that you need to pass your class 12 in the same year or a year before you are giving BITSAT exam. Also you must pass 10+2 with atleast 75% aggregate in Physics, Chemistry and Mathematics/Biology and also atleast 60% in each of the subjects from a recognized board.

Hope this helps. For more information check the link below.

https://www.google.com/amp/s/engineering.careers360.com/articles/bitsat-eligibility-faqs/amp

Good luck!