Can't speak for K0 but for K1, the Yamiya seat is gold... Worth every penny. High quality vinyl, looks correct, very soft comfortable foam. I have a K1 repop seat from the cheap Florida outfit (comes from overseas) and it's a decent seat but it's not nearly as nice quality as the Yamiya and the foam is more stiff and less comfortable, but still far nicer than a 40+ year old original seat with rock hard foam.
If you want the most accurate repop go with Yamiya. If you put serious miles on the bike go with Yamiya. If you want a nice presentable seat that's comfortable enough for a couple hours here and there, the Florida ebay seat is adequate. I thought the Yamiya was ridiculously priced, but again it was worth every penny to finish off my restoration.