Installing New Ecm

I Have Cutlas Ceira 1986. 2.5L 4 Cyl.Some Times It Will Not Start And After 10 Or 15 Minutes It Will Start. Two Days Back It Is Not Starting At All. So I Checked And Found That There Is No Current And Gas Is Not Coming. So I Think We Need To Change The Computer.
Please Let Me Know What I Need To Check??? Your Help Is Appriciated. If In Case We Need To Put A New Computer , Does Any One Know How Difficult Or Easy It Is???

Thanks
Preetam Henric

To start off with, what engine?