In PL/SQL, a stored procedure with a return value is a function:
create function checkcontract
You would call it like this:
l_retstat := checkcontract;
However, if you are moving to Oracle you should learn the Oracle way of working rather than just reworking SQL Server syntax. For example, you could use BOOLEAN rather than integer and use the function like this:
if checkcontract then
... -- Do something when checkcontract successful
Also, if this status variable is being used to check for error conditions, then you should be using EXCEPTIONS instead of status variables.