This is only my second time checking in at the forum since the format change. (When I don't have time to hunt, I certainly don't have time to check out "the forum". I'll see if I can find what I did with the photos of conversions done by others (mostly BJ) and I'll see what I can think of at this late hour to add.
First, I think this is a great boat to convert to a duck boat. You will end up with a fiberglass boat capable of holding a big guy, the dog, and quite a few decoys in the cockpit and under the deck (more if bags of dekes are lashed to the foredeck). It should be easy to pole, row or kayak-paddle. A small motor ,either gas or electric could also be used if an appropriate mount was attached at the stern. A Barnegate style spray dodger or navy-top can be added if you need more than just the cockpit sides to keep water out.
The hull lines of the sunfish have ample rocker and vee making it a good deep water hull. In lakes and bays it would be usable as long as the wind and waves are mild to moderate. The wide flat, "boardboat" design of the sunfish also gives it a shallow draft, allowing you to use it in well under 12 inches of water as long as the daggerboard is not down. Above the waterline, it has the low profile look of a layout boat or a sneakbox.
I can't recall anyone leaving the daggerboard trunk in place. I, on the other hand want my boat to be able to double as a sailboat in the summer just for fun. I plan to move the daggertrunk nearly to the the gunnel on one side and then add another on the other side. A daggerboard in one or both will work much like a leeboard allowing me still to sail it. To have a full size cockpit you can lay down in, I'm pretty sure you'll have to move or remove the daggerboard unless you are very short. If you do remove your dagger trunk, I'd like to get it from you. I would gladly pay you to ship it to me. (I suppose I should follow that with "where are you located?)
Cutting the deck can be done with a skill saw or a spiral saw. Use a resirator; fiberglass, like asbestose can get down deep into YOUR lungs and stay there.
Some folks have had a tough time removing the "tub" (the old cockpit) and grinders or sanders were often employed at that stage. The last time I ran across a post by BJ, he was hanging out at the refuge's boats and blinds forum.
I'll continue to look for the photos; they must be on the retired PC. Meanwhile if you need to contact me, my email is:
k.k.irwin@att.net