You use the motor serial number to buy motor parts. You use the drive serial number to buy drive parts. Again, the boat doesn't matter when buying motor or drive parts
135hp is a 3.0 liter 4-cylinder
175-225hp is a 4.3 liter 6-cylinder
First, welcome aboard
Second, the boat doesn't matter, only the motor/drive matter when looking for motor/drive parts
So what motor and drive do you have in your pontoon?
Your dealer is an idiot, or a scammer
Your motor has non-adjustable valvetrain and roller lifters. If you have a ticking, put a straight edge across the top of the rocker studs to see if one is pulling out.
Low pressure pump moves fuel from tank to high pressure pump. Very common setup
Before spending money, make sure to trouble shoot the system. What fuel pressure are you getting
Buy either a Mercruiser pump or a Carter/airtex (OEM pump)
Do not buy chineseum knockoffs from amazon
Since you are working in water. Get a face mask. You have to verify the cable anchor as mentioned in post #10.
The reunion should have about 1/8" of threads sticking past it if everything else is correct