There is no bad gas to get out, per say. Just feed it good gas........
If it was running, I doubt your fuel is bad.
Check fuel pressure.
Did you change the fuel filter(s)?
Injectors don't tolerate "dirty" fuel very well
You can play with blade design and the cup to change bow and or stern lift, but that’s about it.
Diameter has little effect unless the lower units doesn’t have the clearance for the larger prop.
Pitch is in effect the gear ratio. You can’t go from a 21:1 to a 15:1 ratio w/o increasing the rpm...
What type of pads are you using?
I use Norton Zirconia Alumina 36 grit pads to cut stainless and carbon steel alloys. Use worn pads for finishing work.
Use 80 and 120 for aluminum cut, worn pads and oil to finish.
Need to keep an eye on your hardware and wiring.....
A lot of attention is taken to spec "salt resistant" hardware (316 stainless) and wiring fixtures (tinned) on boats meant for coastal use.
Not so much on "lake" boats.....
If it doesn’t have to come off now and then for repairs, you can use 3M high bond tape.
It is strong and stays where put.
https://www.3m.com/3M/en_US/p/d/b40065787/
I know more than a couple of guys with over 3,000 hours on their 4 stroke outboards.
What was the life expectancy of a 2 stroke built in 1997? Hint….. replaced my first power head at 1,800 hours as PM
Not sure that is the solution…
I have 3 pairs of cut sleeves in my PPE bag. All three pairs are breathable. They would not protect from fiberglas exposure.