The valve seat material is not the issue. Hardened valve seats would be very rough on stainless steel valves. The "rumor" that leaded fuel cushioned valve seats was a myth propagated by the petroleum industry. As far as I know the manufacturers of aftermarket alloy heads utilize cast iron seats. The only better material would be beryllium copper, but that is more expensive so as far as I know the head manufacturers use iron instead. NOT hardened steel.
Hopefully who ever assembled the engine limited dynamic compression to 8.0:1 or less because the cylinder walls may not survive anything more than that under WFO operation. With 8.0:1 dynamic compression the engine should be capable of operating on 91 octane US/Canadian fuel.
93 octane US/Canadian gas is equivalent (or close to equivalent) to fuel rated 98 octane internationally, whereas 91 octane US/Canadian fuel would be rated 95 octane internationally.
So the question is, what's the dynamic compression. If the car comes with a list of parts that went into the engine we could probably figure it out fairly closely (piston part numbers, connecting rod length, crankshaft stroke, camshaft part number, head gasket part number).
IF the engine was assembled by Keith Craft they'll have documentation on file. There is usually a number stamped somewhere that Keith Craft placed there for future identification.
You can also experiment by filling the tank with 91 octane gasoline 5 gallons at a time … operate the car to see if it starts pinging … if not then add 5 more gallons of 91 octane. Keep doing that until the tank is 100% 91 octane. If it doesn't ping then you're good to use 91 octane..