Will do my best to answer with my best guesses. Cool that you have a Barbour, are you in NC? Know they were made in New Bern.

1. 50:1 Tcw3
2. Doubt the blade is causing the vibration. Not sure of severity, but could be “dead range” of motor as it climbs from idle to throttle. My OMC big twins tend to have a flat spot in performance at a specific mid range. I can minimize with tuning carb needles/ensuring timing and carb synchronization is right. Not saying that is it, usually not a bad vibration as much as it is a noticeable sound and shimmy. Beyond that check motor mounts and make sure they are still solid, rubber could be bad,
3. Ebay or free classifieds here for prop
4. Not sure if you are running metal or plastic tank, but I assume it is portable. If plastic make sure it is vented (metal tanks too, but it is built into cap). Did you replace fuel pump diaphragms? If not they probably need to be changed. Finally, I have an old Merc quick silver tank in my whaler that has a “reserve” area in its shape, and if I have it turned wrong direction in the boat, it will drain gas in part under the pick up.