So, I've been a person like this on a smaller scale (taking a fully written program, and basically "completely rewriting it" in a new language. Biggest thing I can say is that if you have enough of a "crazy-eyed mofo" that's just insane enough to take a big job like that, 3 things are of utmost importance:

1] remove their technical blockers. Often a person like this doesn't have a universal skillset — they're usually good, but they can't do everything. If there's a 5% of the job that's hard-blocking them, make sure it gets done so their tires don't get stuck in the mud.

2] remove their bureaucratic blockers. Make absolutely sure the project WILL proceed, even in a skunkworks capacity (this is the primary value of skunkworks — it's the ability to proceed with something you know will work, in spite of authorities expressly forbidding it because they think it won't work and don't want it to happen for what's usually a petty reason like a "it's waste of resources").

3] remove their emotional blockers. This is really the apex — THE primary value of a person like this is not technical skill, but rather, their work ethic. A person like this has a really profound, train-engine drive to just keep soldiering on. The danger here is the problem of the "weary crusader". This willpower is considerably above average ... but it's not infallible. It's not like a superhero that's always gonna come through no matter the odds. They can break down. They can lose heart.

One of the things that will constantly break them down is if people are naysaying them, and calling into question the value of their work. If they're doing a giant refactor, and people are angrily opposing it simply out of fear of change, it will break them down. As silly as it sounds, you want to coddle them — they might be unusually emotionally tough, but they are the absolute last person you want breaking down. Treat them like a "snowflake".

And yeah, I'm explicitly suggesting censorship. Don't permit negativity. Don't permit the usual cesspool OSS discussions where you have a bunch of people who contribute almost nothing bikeshedding a project to death and exercising a sort of "liberum veto" on any attempt to do new things. Just shut down those conversations — make it clear that only the people who do major heavy lifting (or really, who are discussing things "in good faith") have a voice. The whole OSS community has an unhealthy cultural fixation on "absolute free speech", and like ... I completely understand where that comes from, but it's absolutely toxic for motivation. I've observed over the years that a lot of the more aggressively successful groups at "creating things" basically just establish their own safe-spaces where people get emotionally reinforced instead of emotionally sabotaged. Occasionally their work will get exposed to the world and some toxicity from outside will leak in, but the bread-and-butter day to day experience of working on their projects has them surrounded by friendly people who believe in what they're trying to do, and encourage them to keep going.

Partly because we know we won't be there forever to keep working on it. If we feel like we're part of a group that "has our back", and will support what we built, and grow it into something, it's a million times easier to keep working on something, compared to a situation in which someone actively holds our work in spite and is itching for the first chance to tear it down the moment we turn our back, or move on, or die, etc. This is all "emotional extrapolation", but these are the sort of depressive/anti-depressive thought cycles that go through your head during a mega project, and too much of the negative side can just break people.