You MUST MUST have a fast acting Rev Limiter !!!!!!!!!!!
Before taking the boat the first time !!
My stock Chaparral 19' With a stock 305 Merc leaped out of the water on my first WOT test run. About 1.5 seconds air born The stock Rev Limiter saved the engine........... Calm day, no boats...
You have a REAL PROBLEM with locating where to put the transducer. Tunnel hulls have loads of air along the top of the tunnel at different speeds. I would mount the transducer 10" to 12" outside of the tunnel bottom edge. Near transom Where the hull is pushing down into solid water at high...
If you have a BIG car battery SIZE like larger than a size 27 ?
You need to fully charge the battery with a real normal sized battery charger. It could take several hours at10 to 15 ampsCheck water level in each cell. add if needed.
Any old motor should have starting or running problems. That is why it is for sale. Buy for 75 to 125 maximum. If parts are still available by you looking for them.
The LEGAL owner of the boat has legal responsibility for giving the unsafe boat to the person. Does it have enough USCG gear on it. Let the 2 insurance companies pay for all the damages.