There is no USCG requirement to have a fixed system in pleasure craft.
Halon is one of the best fire suppression agents. The manufacture of Halon ha been stopped because of EPA regulations. But it is still recovered from major systems and is available for refilling.
All you need to do is have it...
Look in auto parts Dorman expansion plugs or boiler plugs.
Quick temp solution that you can use to be able to run the engine and test for any larger issues. Some people just leave them instead.
Out drives have flappers in the exhaust pipes behind the engines.
Inboards do not normally have internal flaps.
Some boats have external flaps on the exhaust ports at the transom. To prevent water ingress when backing down hard.
Do you have thru the bottom exhaust?
First give us all 8 cylinders.
Next 2 & 3 are adjacent so could be blown head gasket. But after 4 years more likely stuck valves.
If you want to continue next diagnostic would be leak down test
I would start by soaking the cylinders with some oil first. Loosen the rings and clean surface rust...
The cleaner is fine.
Be gentle disassembling and reassembling.
Do NOT rough the contacts up.
Rough surfaces will cause more arcing.
Most marine switches are silver plated sanding will remove that.
A old toothbrush or small stainless wire brush should be ok.
If all ready pitted then a fine 800...
First to be sure we are talking about the same thing.
I/O with Mercruiser Alpha drive?
First check that the shift plate at the engine is moving. Could be a cable issue.
If that’s moving check the second cable from there that goes to the outdrive.
More often cables break than the drive.
Let’s break down what we are talking about.
As mentioned the internal antennas will vary in performance and accuracy depending on where the device is mounted.
Most MFC/MFD have NEMA 0182 and 2000 outputs for other devices.
Such as Autopilot or VHF DSC radios. Some VHF DSC radios have built in...
Check how each manufacturer rates the “size”
Marine trailer winches are often rated for the wieght of the boat, not line pull.
Standard winches like truck and ATV are the line pull in pounds.
Take the tap down to a local pawn shop. They often have bins full of misc sockets. Find a twelve point socket that fits the tap. Then use an extension to be clear of the rams.
As stated aluminum is easy to tap.