For brakes, we never use copper tubing because it work-hardens under each foot-press, and often cracks. But that's under about 1100 psi from the power booster to the brakes. For the clutch, the pressure is only 2-300 psi so copper tube will work safely. And the difference in OD between U.S copper and Euro is small enough and the pressures are low enough that running the proper drill thru a stock bubble-flair fitting should re-size it well enough. Never actually tried it, though. All my flex lines are Aeroquip braided stainless.
On the bell housing end, there's a metric thread 3 way junction that attaches the red plastic flex line. Those stock threads are sufficiently close to SAE dash-4 that a std dash-4 AN fitting will screw right in. I add a flurocarbon o-ring as insurance. Then you can use SAE braided stainless line up to the slave cylinder. Most slave inputs nowadays are threaded SAE 1/8" NPT (or can be made so). So a 1/8 SAE NPT-to-dash-4 fitting will connect the other end of your stainless flex line. After 50 years of soaking in brake fluid, I'd expect the stock red plastic line to be very stiff and brittle, maybe ready to vibration-crack.
For the benefit of the next caretaker of your car, I suggest painting any altered fittings like this red or yellow if you ream the euro fitting for SA line size. And jot down such changes in an "As-Built" notebook that should always stay with the car. It's hard enough to sort one of these things out when the former owner is still around to answer questions. That's not a 'given' any more....