Using another small engine as a model: the Triumph 650 (40" twin) has 1.50" OD (1.40" ID @ 18 ga.) primaries. This is not merely sufficient, the bike is slower with any larger tube up through 800cc (24" cylinders) and 60 hp. A 1.75" "TT pipe" kills power below 4,000 RPM.
That's 180 hp as a 6 cylinder...
The engine will probably be more tolerant of long overlap at low speed if the high exhaust exit speed helps kill reversion.
If I were offering advice on constructing a tube header, I would suggest using the largest possible radius tube leaving the flange (I found 3" radius), even if it must turn 100 or 110 degrees (back on itself to avoid steering, etc.) to get the best possible flow.
