That conversion is very nice - I took him for an Avar at first because of the horse armour and face mask, but then, no stirrups, which is the big thing we have to thank the Avars for
Stirrups could easily be added, though it's a shame the horses aren't available separately. Fireforge's Byzantine Cataphract horse sprues might be available separately, but the barding is lamellar, not scale.