Help how to replace drain assembly

[teamprine]

hey there hope someone can help me I've got a photo of my drain assembly for my one piece tub it's completely disintegrated no idea why!
How do I replace this it appears there might be some threads down there but I have no idea how I can unscrew it
As you can see from the photo portion that kept the plug in place is completely disintegrated as well
Tips on what to replace it with would be great I'm usually pretty good at DIY but I wanna make sure I don't have to disassemble the whole tub or anything!
thanks

 

[havasu]

Do you have access to the underside of the tub?

 

[teamprine]

Do you have access to the underside of the tub?

no i do not it's a one piece tub shower on 2nd floor

 

[Geofd]

There is a tool that expands inside the drain or a set of easy outs