I don't know either. I couldn't find anyplace where he might have given instructions on making it. Looks like the motor is up in the housing at the top. It works really well.
Here is another excellent DIY underwater treadmill done by someone else.
https://www.youtube.com/watch?v=s8kZ5ZTHszc wrote:Materials: (Putting this list together after the fact)
The treadmill is a stripped down Sears free spirit treadmill.
Qty 1 - Temp controller Amazon
Qty 2 - 500W aquarium heaters, Amazon
Helpful photos are here, including before and after photos showing the motor where it was originally in the base of the Sears treadmill, and then he installed it up at the top of the tank of the underwater treadmill.