saying most/all projects trying to use another engine failed, can happen because of a numbers of factors.
-limited programming/engine knowledge.
-badly interconnected system making them hard to understand and/or use
-too much focus on having 100% replication of how FF7 did it.
-loss of interest/little interest shown(makes the project seem not worthwhile to complete)
Although not the same game, it still applies.
Afterall by comparison before I kinda lost the interest in my own programming something to emulate FF8 was simply time consumption but in approximately 2 months thanks to all data/information/tools for FF8 I implemented.
-Full angelscript real time compile for whenever I changed a player's limit break damage calculation or spell damage/heal/effects
-Behaviour Tree based battle AI
-Spell and junction system(both extremely flexible and capable of executing each their own damage/effect script)
-GFs and Limitbreaks
-All monster stats and level/stats scaling
-world exported from FF8 converted to heightmap then added to terrain system(3D).
-2D backscreens with 3d player ontop(as expected as a copy of these games, with multiple layers support allowing animated layers to be grouped,like curtains blowing the back and stop(emulating wind has stopped))
-Character UI, party switching, junction system, item list.
-Worldmap battle encounter, regional monster tables
Again this relies too much on
-how much time you are willing/can spend on it
-how much knowledge you have around programming and what areas you are capable of finishing.
-You have others helping you in the areas you can not do yourself
-good design and readability of your code(doing solid prework and ideas of the systems can help alot in how you will connect them)
lines of code for programmers means absolutely nothing, it is what those lines you have does which matters(I've seen alot of people writing huge amount of code which could've been handled in less than half the lines).
using opcodes and guesswork on what to change to see if it breaks or not, to me sounds like a hard way of programming when going to replicate/reversing projects.
Lets say on average the each field has 1500 lines of code which is generous. Multiply that by 700, and you got 1,050,000 lines.
That is asuming you need 50% of the lines avoiding duplicate functions/code in the scripts(which probably almost every field shares in someway).
KnifeTheSky77 is correct about Unity.