Re: 14' Duracraft transom replacement
Nope, don't be surprised that the transom is original.
It may not be, but it's equally as likely that for a 1981 jon boat type 14ft'r, Duracraft may have built it exactly as you see it.
The aluminum skin, in general, is typically thicker on welded Duracrafts then on similar riveted built boats. Particularly on the transom skin. Even a few thousandths can make a big difference.
Everything you're seeing & describing is nearly identical to a 1957 Duracraft runabout. It's transom wood isn't full height, 12"+/- (short 6-7" of reaching the bottom of the hull), doesn't extend all the way to the sides of the hull (1-1.5" short), knee brace thru bolted at the transom & riveted to the hull bottom.
he exception is it doesn't have the small lower shelf to hold the bottom of the transom plywood. It has a 1958 Johnson 'Fat Fifty' 50hp OB hanging on it w/ nearly zero deflection & the transom ply is VERY soft, 1 step up from complete mulch. That motor is extremely heavy, compared to a 1971 650 Merc 65hp motor that is also heavy, the FatFifty is HEAVY.....
If it were up to me, I'd probably consider removing the shelf & attempting to slip the new transom plywood into place behind the knee brace.
Before that I'd get everything else I intend to remove, removed: motor, electric trolling motor, etc, and confirm there are no leaks @seams, welds or rivets. Based on those results decide how you want to proceed on the interior: Do nothing, repaint, rehab, modify etc.
If there are leaks that may dictate how you proceed. And depending on that, decide to remove the shelf at the bottom of the transom or not.
Once a direction is chosen, you'll get all the help you'll need to put it back together, new rivets & all.