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I just received the MM-5 Stirling engine and put it together. The
first time that I ran it, it ran like a champ. After doing several
trials with hot water and a 60 watt light it was set aside for two
days. Tried to run it again and the performance is very poor. It
barely makes 4 minutes on hot water and it runs very slowly. I have
retested for an air leak, and it still passes that test. I do not
know what to do and my son needs to demonstrate it for his science
fair project. Please help!

Obviously this state of affairs is quite frustrating.

If there is no leak per the leak test then something else must be the cause of failure to run. You should be able to flip the propeller and the engine turn over approximately three revolutions. If not something is dragging or preventing free movement of the displacer and/or connecting rod. Check for interference between the displacer disc and the plastic cylinder. Is the displacer hitting either the upper or lower plate.(Most likely disc is hitting upper or lower plate or less likely that the rod has been bent.

Further suggestion is to follow the assembly procedure steps for final adjustments prior to running the engine.