I also use muriatic acid and it works good and fast. I get a small bucket with water then add the acid to the water and use a cheap paint brush to apply it to the props, then rinse it off. Make sure not to add water to the acid and wear gloves.
Here is a great place to look for all info about Lake Powell. http://www.wayneswords.com/ We stay at Wahweap when we don't stay up lake at a beach. We have had friends come and stay at the camp ground there while we stayed on the boat in the slip and just went out for the day, it is a nice camp...
It sounds like you fixed it but if that does not work out have you tried ebay? Here is one that i think is the right size. If the link does not work just search taylormade hatch on ebay...
I have seen alot of 268's with 3 deck hatches. I have even seen a few 250's with 3. I also cant really think of any other searay from that era with a foward facing radar arch.