I am a Kagi supplier too. Although I stopped selling with them some time ago, I still have a lot of customer data in their database. I was surprised to hear of the closure and received no notification from them.

I have been using Share-It for over five years. They have a solid system for running licence generators and although it's not the most modern, I can recommend them as an equivalent service to Kagi. If you sell in the EU you'll want to check the fine print on their VAT policies.