All you will need to fix your troubles is an abandoned Aldi box from which to cut the back (it can then go back to housing dolls and blankets and other such items, no harm done). Cardboard in hand, you will need an enthusiastic parent who is willing to dedicate a large portion of their morning to meticulously blocking of a half inch grid on said cardboard with ruler and pencil. In the mean time, take a paper and pen and write down the people, weapons and rooms used in the game of Clue. (Be sure to spend adequate time arguing over the rooms and weapons as not all games include the same names for these items!)
While the parent continues the intricate task of half inch squares, have one child cut 2'x3' cards and write the names of the people, weapons and rooms that were finally decided upon. Have another child make the papers to keep track of all your info while you play the game. Rob your Careers game of its dice and playing pieces. Fashion an envelope to place your suspect cards in.
Once the parent completes their tedious task, cut squares of paper to represent your rooms. Count the squares on your grid to determine how big these rooms need to be. Glue rooms in place and write their names in large letters. Mark the spaces for each playing piece and doorways to each room. At this crucial point is a great time to think of the idea to google "Clue playing board" to see a picture of how the board actually looks. Having done that, you may or may not discover that you have added an extra room to the game. You may, a) say "forget it" and play the game with an extra room or b) peel two rooms off the board and make one new one to take it's place, it all depends on the people in your family.
You are now ready to set up your game, deal your cards and let the fun begin!
P.S. To the parent: instead of focusing on the fact that it is now noon and you will need to make lunch soon, comfort yourself with the thought that you have saved yourself 99 whole cents at the Goodwill, and that the game you saw probably didn't have all the pieces to it anyway!