Here are your options:
1 - Use your own family and recreate your research calendar. The drawback here is that you did much of the research years ago and with your current level of experience you may now approach the same problem differently. Reviewers will be looking at the order you searched items and the methodology used. After many years of experience your searches will now be more direct, you have a better understanding which records would give you the answers you need. Also I have found that when researching our own lines, there are emotional ties that sometimes cloud our good research judgement.
2 - Use a recent client project. This option is best. You can showcase your best work in a recent project. You will not have to reconstruct anything. And you will be using current resources. The ICAPGen written exam is designed to test the competency of a genealogist who researches for clients under a timed constraint. This type of project will give you the best preparation for the written exam.