I played the first Fallout right after I finished FO3 and I thought it was simply amazing. I also loved New Vegas, but I still can't get myself to finish FO2, I'll eventually do.
I've never, ever, had the faintest thought of trying to translate either games to their respective engines. Now that you mention it, it could be quite interesting, to say the least.
I think it could be possible to translate each games to the other engine doing some changes into the core gameplay, trying to retain as much as possible, although, storywise, I can see FO3 as the weakest of the bunch as a 2D isometric RPG, but New Vegas would certainly be toe to toe with it's older siblings.
As for translating FO1 & 2 to the newer engine, I think it's entirely possible to translate the entire gameworld on it, but I can also see a lot of empty space in the first game, so it could be filled with whatever you can come up with to fill that gap, if the changes make sense of course. In fact, I think it would be better to build entirely new places altogether, having the first two games as a foundation. "Reimagine" them, as some would say. Others would call it a remake entirely.
So yeah, to summarize, it could be possible, but some changes would certainly be needed to accomplish such a ginormous task.
Also, I don't think FO3 ruined the lore in any sense, I still played it after the first game and I could still appreciate the game even more than before, now that I got a bit more of backstory. So yeah, FO3 is my favorite and my only reason to still having GFWL installed, but I still love the series as a whole. Tactics is still sitting there in my Steam library, collecting virtual dust. I didn't liked it.