We DO modifications to aircraft at the Planes of Fame and it just ins't that difficult. Someone is making mountains out of molehills.
You don't have redesign the entire rear fuselage ... you have to make the top curve shorter and redesign 5 - 6 bulkheads to have an abbreviated top curve, change the length of a few stingers (VERY simple), maybe a couple of longerons and maybe not, and resahpe the upper part of the skins.
The fin and rudder always could have been made a bit bigger and had trim added to the rudder.
From someone who does restorations and modifications, it isn't all that big a deal. The ONLY reason I would not do it today is becasue any Messerschmitt Bf 109 I work on should be a s stock as possible. Back in the war, it would have been a VERY different story.
They didn;t seem to have much trouble doing it to the P-051 Mustang, the Li-61 / Ki-100, the Spitfire, the P-47, or the P-40 (think P-40Q). Since EVERY ONE of tehse had the same thing done, how tough is it?
Answer, not very. The first trial example could be done by 3- 4 guys in 2 weeks or so with some plans (fuselage changes only ... someone else would have to make the canpoy and slide pieces.
A friend and I made a piece of trailing edge about 4 feet wide for a North American O-47 last weekend (port stub sing) from nothing but Aluminum sheets. We cut the 3 ribs, formed them, cut the top and bottom skins, made the stiffeners and a few braces, and drilled enough holes to hold them together in 8 hours on a Saturday. We took our time and will have this piece of trailing edge done in about a total of 3 Saturdays with 2 guys who are working at this for fun and not as a job. We shoot the bull while working and keep moving, but not at a breakneck pace ... it IS for fun, after all. The main issues are having the material, tools, and plans. We traced the original parts and went from there.
This just is NOT all that difficult as is being touted in here. Ask FlyboyJ ... he works on real planes, too and can do sheet metal.
When they punched a hole in the horizontal stab on Rare Bear at Reno on 2011, they had it pactched in 12 minutes. Applying the bondo, fethaering it in, wiping it with a tack rag, and painting it took longer.
Modifying an existing fuselage would take longer ... making a new one from scracth would be WAY faster and easier, and might or might not save time from the existing design.
It's sort of like turning a right hand Allison engine intoa left turn engine. It is a major change if you have an existing right-turning engine. But if you are building one up from parts, it is trivial to make a left-turning engine instead. There is a difference of 3 - 4 gears, a different wire harness for the spark plugs, and an opposite-turning starter. If you are assembling it from parts ... the difference is miniscule.
If you want to CHANGE it, you have to pull the engine, remove the accessory case, the nose case, change the starter, and change the wiring harness ... and, of course, the propeller. Time-consuming. But build it from scracth and the difference can be neasured in minutes ... the length of time to read the changed instructions for sequenced assembly.