A few months ago, searching the Web, I found a switching power suply replacement for the hot 7805, the
DE-SW050(Dimension Engenieering). It is only a little bigger than the 7805 IC and gives better characteristics whit the same pinout.
As a proof of concept I've installed it on a Spectrum, discarding the heatsink!, and it now works perfectly at ambient tempeature...
I've also replaced the regulator in my EXDOS card, now powering the HXC Floppy Emulator.
For now I will not change the internal regulator of my EP, as I fear for the keyboard membrane keyboard. (I already broke them on the Spectrum and had to put a new one)
On the other hand it cost each 15$ (+1.25$ shipping) but I think is a good investment in our loved machines to get them more cold....
This is the Web page of the company where they sell worldwide:
http://www.dimensionengineering.com/products/de-sw050