From my view at 30,000 feet, it seems like you're looking for trouble. If this machine works as-is, I'd say leave it alone and use it for 5-axis work on light materials. If it doesn't, then fix it until you can. If you want a larger 3-axis router for cut-out work on sheet goods, that's an entirely different machine, with a lower Z axis. Unless you scrap most of this thing, you won't get that out of it, and you'll probably find that what you saved from it wasn't optimal for the task. It sounds like you've got enough talent around there to build the router you want from scratch, and it would probably be a better machine than whatever you could manage to make out of this one.