I replaced the aft pilothouse-main cabin window with a custom window from
Motion Windows
They will custom make windows for about any opening, just a matter of cost.
You can get replacement aluminum window slides here:
http://www.marinette.com/
As far as the water tank goes, I wouldn't drink water from a 30+ year old metal tank either. We flush ours every spring, and throughout the season and just use it for washing, etc. We keep bottled water in the fridge for drinking. We would probably do this regardless as we just don't use enough water from the boat tank to keep it fresh.