Next hard-to-decode panel.
そう 本来なら。 だが20世紀後半 に一世 を風靡したという外食チェーンを 模し
たそのレトロな 店 だけは別 だった
そう ほんらいなら。 だが 20せいきこうはん に いっせい を ふうびした という
がいしょく チェーン を もした その レトロな みせ だけ は べつ だった
sou - yes
honrai nara - by all rights
daga - but
20 seiki kouhan - second half of the 20th century
ni - by
issei - lifetime, generation
wo - thing being acted on
fuubi shita- dominate + past tense of verb "to do"
to iu - called thus
gaishoku - eat out
cheen - chain
moshita - past form of "to copy", "to model"
sono - that
retoro-na - retro-ish
mise - shop
dake - only
wa - subject marker
betsu - different
datta - past form of desu, "is"
yes . by all rights.
but . second half of 20th century . by . generation . dominated . called thus . eat out . chain . to copy . that . retro-ish . shop . only . (subject) . different . is
"Well, by rights. But, there's one shop, that copies the retro look of the so-called fast-food chain that had dominated the 20th century, that is different."
To break this down, we have to realize that the hero is talking about three things: first that there was a fast-food chain that dominated the 20th century (presumably McDonald's); second that there's a shop that boasts of its paintings that also copies the retro look of the fast-food chain; and third that the author considers this one shop to be worth entering.
Which gives us:
"a so-called fast-food chain that dominated the 20th century"
"a shop that copies the retro look of the chain"
"But, I consider this one shop to be different".
Put them together, and this is what I went with, "By rights, yes. But, there's one shop, copying the retro look of the fast-food chain that had dominated the 20th century, that is different."
It's still a bit long and awkward, but it has both the intent and feel of the original Japanese (which is also long and awkward for most native speakers.
To be continued.