Hello,
I am not sure if you already use the custom security handler (
Custom security handler documentation). If so, you can use all standard web parts (i.e. Logon form web part).
Only the “Send password” functionality will not work. However, you could modify the web part (
Modifying the code of standard web parts) and add the functionality (which will work with your external database) by yourself.
Best regards,
Helena Grulichova