if the coupler is bad it is usually from not checking alignment every year
you do pull the drive every year to check alignment and u-joints per the manual, correct?
common things that lead to the drive being out of alignment:
rear mount bushings fail and motor sags
transom is rotten and motor...
the clamp-on cable terminals are junk. only way to clean a cable that is corroded between the strands is to replace it.
get a set of properly made cables with properly terminated ends. I usually make my own using marine battery cables (tinned copper)
agree on compression numbers
generally if the timing appears to have moved that much, usually the outer ring of the damper is no longer bonded. use a cylinder stop and degree wheel to indicate TDC. if the damper ring has shifted, get a new damper.
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.