Are the tube diameters and heads the same on the D and Eb?
Yup.
Best wishes,
Jerry
Great answer! 
Short and semi-sweet. I bought an Eb Gen the other day. I swapped the head with an extra Cillian O’Briain Improved head. And I put the Gen Eb on the Feadog tube, it works great.
I’ve read many complaints about the Gen D, and much praise for the Eb. I guess I just lucked into a good plastic part.
Some other good swaps I’ve found is putting the Walton’s LBW head on the Clare body. The fit is a bit loose, but nothing a little tape didn’t cure.
Most mass-produced soprano D and Eb whistles have interchangeable heads.
I’ve only seen Eb offered by Generation. What other mass produced Eb whistles are available?
I seem to remember it as Clare or mabey even Feadóg, but I’m not sure? It’s been a long time since I bought any soprano Eb whistles.
I know that Susato produces Eb whistles, but they’re not the metal body-plastic head type.