No recommendations for alternative iron unless the vendor has one. It would likely require changes to the feedback circuit that would require some knowledge/skill to perform. There are reproduction ST-35 transformers being made and perhaps the vendor simply felt they didn't justify the cost or weren't available when they designed the kit?
Since the amplifier is autobiased the largest issue using the toroidals is eliminated. It may be worth reaching out to the vendor to ask about output transformer upgrades. If the circuit is close to original, it might be reasonable to swap in the reproduction OTs since you've already invested quite a lot in other components most people building these wouldn't.
