Using LIKE in a For Loop with UpdateCursor

I would isolate the class you are after rather than searching for it. Then, simply assign row[1] that isolated value. For example:

in_table = 'l12wm_agsup02.img'
field_names = ['Class_Name', 'crop']
with arcpy.da.UpdateCursor(in_table, field_names) as cursor:
    for row in cursor:
        row[1] = row[0].split("-")[2].split(".")[0]
        cursor.updateRow(row)

This does not have to be done in a python script, it can be done simpler by using the calculate field tool as shown below:

If you want to run this as python the code would be:

arcpy.CalculateField_management("myRaster","crop","cropnumber( !class! )","PYTHON_9.3","""def cropnumber(s):/n  if s.find("-1.") != -1:/n    return 1/n  elif s.find("-2.") != -1 :/n    return 2/n  else:/n    return -999""")