Exhaust elbow: It was with the boat when I bought her. I simply cleaned it up. It is Onan P2833.
Timing: Yes, I'll recheck/adjust timing when I launch in the spring. I really like using a timing light to take out some of the guesswork. I've attached a pic of rather crude timing indicator installed next to the auxiliary pulley. I punched and painted a notch in the pulley that references TDC of piston #1.
Adjustable main jet: No, I don't have one. I'll revisit that.
Indigo thermostat: I'll let you know. I bench tested without temperature gage and could definitely tell when it opened. The bypass loop is quite sensitive to air pockets so I have to be careful to purge air otherwise exhaust gas doesn't get cooled when the engine is cold. I melted my makeshift exhaust hose. It was sump pump hose so it didn't take much heat.
Indigo PCV: My engine has a history of "exhaust fumes" escaping out of the engine compartment. Turns out the shifter cable was pinching off the rubber breather tube which allowed crankcase fumes to escape out both oil fill tubes. I didn't catch this until I removed the breather tube in my garage. It was actually torn. I had ordered the PCV kit by then so I just went with it. Added bonus of the PCV kit are fuel hose and fittings to replace the copper loop connecting the mechanical fuel pump and carburetor.