I was also toying with the idea of making new shafts and hubs to ~original spec, with obvious modifications like shortening the splines and using 30mm bore bearings on both ends, but also using better material and hardening the splines. Maybe a different spline profile if deemed necessary, but otherwise drop-in for the Europa. They really should be replaced as a pair anyway.
It may turn out that a fundamentally different layout of the components is necessary (moving the splines to the yoke side, as seen in some unobtanium designs), but I wouldn't go so far as changing the u-joint size or otherwise messing with the fundamental Europa suspension design.
We have a fairly similar part at work that should serve as decent price comparison (no u-joint yokes though), going to find out what those cost for the small initial batch. If the cost is ~$800 per car at qty 6-10 I'll likely put in the effort to do that.
If anybody is reading this in a few months, please bug me to update. I'll get to a y/n decision on feasibility and make a fresh post.