It is likely oxidation. Get some FSR (fiberglass stain remover) from West Marine or someplace similar. Wipe it on with a sponge, wait about 15 seconds then rinse and wipe off. Having said that I typically see yellowing near or below the water line and not top side. This stuff won't hurt...
For some superb food try Marlow's Tavern. Fantastic chefs at a reasonable price. For a great all around sports bar Miller's Ale House., nothing fancy but lots of space, TVs, beer and food, and open late.
The fact the the oil sat in there long enough to separate means it also sat in there long enough to corrode parts of the inside. If it were me it would be a full rebuild.
Like boat doc said, get some wood and make a gantry. I made one 12' high and 12' wide. $120 worth of lumber and hardware and about $60 for a 1 ton chain fall. Worked like a champ.
Adding wide band to boats is easy. I've been running it full time for the past 7 years. Sensor location is the key. AFR feedback is priceless. The gauge took the place of my fuel gauge.
Do not use the slip in silencer you showed. I have a pair sitting on the garage floor. Those slip ins do reduce the sound about 6 dB but on my small block choked WOT rpm by 300-500. With a 525 it will be even worse.
Get the GGB inserts. They make several types and sizes. Get the...
We had the smallest one, the 50' "Forever" model. I think it's actually 59' long. We had 4 people. Perfect size for that group. It sleeps 6 or more but I would not want more than 6, but I'm picky. I was pretty happy with the boat. Biggest gripe was not all the windows had screens and the...
How many people are going on the boat? Don't get a boat too big if not needed.
I would recommend Antelope point, not so much that it's closer to uplake stuff, but the people there LOAD your gear on the boat for you. They had us loaded and unloaded so fast my head was spinning. Very fast...