You could use Azure Logic Apps or Azure Functions.
For most scenarios where existing Biztalk systems need to be replaced, my Biztalk MVP colleagues are recommending Azure Logic Apps.
I'm a fan of Azure Functions but they don't offer the built-in options that you get with Azure Logic Apps. Conversion will still be a large task, but you'll get there faster with Logic Apps.
One note: I see people trying to do with with Power Automate. Don't do that. It's targeted more as an end-user tool than a server tool. Logic Apps is a server tool.