Harlequins forward Leonard has missed England's last two games after suffering a hamstring injury during the 25-17 victory over France - his 100th Test match appearance - five weeks ago.
Northampton star Cohen sat out the 40-5 success against Italy eight days ago due to a thigh strain, but - together with Leonard - he has now successfully completed his rehabilitation programme.
England also believe second-row forward Danny Grewcock will be fit for the Calcutta Cup encounter, despite suffering what the Rugby Football Union termed as "mild concussion" during his club's Zurich Premiership game against Northampton.
Smith - capped as a substitute against Italy - is, though, sidelined.
The 20-year-old suffered a shoulder injury when Leicester lost to Newcastle and will be treated by the Tigers' medical team this week.