Repairing the carburetor takes very beer tokens and no time.-----The total teardown of the powerhead is going to take a lot of both.-----First see if 3 new pistons and machine work is available and in your budget !!.----Pistons may be hard to find.
Looks like BC.----Determine condition of running engine.-----Determine condition of the drives.-----If good then rebuild / repair the bad engine.----Sell the boat.
I could rebuild that motor with hardly a glance at a manual.-----I think you need to learn a wee bit of trouble shooting !----Good luck with it.----Note ---I own about 10 or 12 inline 6 Mercury motors.