I bought a KKK long tail mud motor from my buddy and he told me it ran flawless. Come to find out, it doesn't. The motor will fire right up with no issues. However, whenever the prop hits the water (spinning), the motor dies. What is wrong with this ? Can anyone help me out or point me in the right direction? Please and thank you!

Try disconnecting oil level sensor. ITs a common flaw in clone engines.

I had the same issue, if it's the sensor killing the engine ad a bit more oil. If you spring a catastrophic leak you'll wish for that sensor, but overfilling just a hair isn't going to hurt anything.