As of my research and knowledge, this is not a generally valid statement. The action of switching a software project from a 32bit to a 64bit (and vice-versa) architecture is a matter of few minutes/hours. You just have to specify the compiler/interpreter in which architecture you want to build the project. BUT, even if the compilter/interpreter managed to create a build, that doesn't mean the game will work fine.
I have no in-depth knowledge of the source code and compiler myself, however I can say this with absolute certainty:
I've first played RoM during the last week of its open beta in 2009 before it was released. March 11 to be precise, it was my first MMORPG and I've grown incredibly attached to it. So much so that I stayed with it until the summer of 2012 when ArenaNet released a new game at the time. I've seen the infamous crits myself, I was there when Chapter 3 was first released with Dalanis. I've been through the Hall of Survivors that too caused a great many crashes.
I've also dipped into the private server community ever since I first picked it up because I found the way MMORPGs work very interesting. I've seen servers for th is game, for Aion and others and also used them. Although in the case of Aion, only as an emulator because the official server files were never made public.
However, a 64bit version of RoM does exist. That wasn't the first time there was access to RoM's server files. I've seen and used them before 6.5.2 and after. I've spent a great many years trying to find this stuff because I could see the game's overall downfall way back in 2014 and witnessed too many MMOs shutting down for good to risk losing yet another one. Tera was the most recent game. The 64bit version I got to try led me to experience an entire 14 days with 8 hours of constant playing without a single client crash at all. Obviously this might be different on a server with thousands of players on it.
Gameforge is aware of the existence too and that is exactly why certain terms are on the censored words list here. I've openly talked about this matter with a few moderators as well and never denied what I gained experience with. The nature of this is also why I'm not divulging any more details. It is a very sensitive matter and many people at GF don't like it for good reason. But its existence can't be denied.
Instead of combatting it, IMO it would be better if GF officially acknowledged it and made attempts to purchase the RoM IP entirely and acquired the files for themselves so that the game could exist officially in a polished, modern version and with some more edits regain a bit of its former fame.