Red Samurai armor is statted up in Arsenal. Nothing for it in 5e.
The Arsenal version was slightly different than the regular milspec armor.
Modern Oyoroi Armor: 11B/11I Armor, Capacity 11, 18F Availability, 11000 nuyen. (Roughly equivalent to light milspec armor.)
Red Samurai Oyoroi Armor: 13B/13I Armor, Capacity 12, 20F Availability, 15000 nuyen. Includes fixed Mobility 2 upgrade. (Reduces encumbrance penalty to AGI and REA by 2 and adds 2 to Running.)
Kabuto Helmet: +2B/+2I Armor, Capacity 4, 10000 nuyen. Treat as military helmet.
So someone wearing a properly adjusted set of Red Samurai armor with the helmet (15B/15I armor) could have a Body of 4 and not take any penalties to AGI and REA, since milspec used Body x 3 instead of x2. 3*4=12, and the penalty scales at -1 per 2 points (or fraction thereof) that the armor exceeds that.