If the boat were travelling down the hi-way, air moving across the engines from the vents would accelerate the heat transfer through convection. They would never drop below ambient (30°F). To tell how long transient heat transfer takes to get the engines to 30°F, you need to know the velocity of the air (at 1 atmosphere) moving across the engines. It will be a natural convection type of problem. We also need to know the conduction coefficients of the iron, and convection coefficients of the air (both can be looked up in tables). You could neglect the paint thickness effects on heat transfer as this would be minor. Also need to know the initial temperature of the engines.
Knowing the thickness of the block or head wall will give us the time that the INSIDE wall of the piece will reach 30°F. From there, we still need to calculate how long it will take the water to freeze at 32°F. That becomes a thermodynamics problem that we will need to develop equations of state as there are phase changes in the water (liquid to solid).
That said, we can spend a lengthy amount of time gathering info and doing calculations... I think the boat sat outside for a lengthy period of time. Someone is not being a "straight shooter"