The transformer most often mentioned is the Magnequest TFA-204, and all but the very earliest OriginalSEX amps were drilled for mounting that transformer. Bottlehead never supplied it directly; but you can still purchase them from Magnequest
http://www.magnequest.us/There are of course other mods that look desirable from today's perspective. The amp went through a number of variations, so any advice on modifications beyond the transformer transplant would start with identifying what you have. A clear photograph of the underside and topside would be a good place to start!
My personal OriginalSEX amps have not been in use for more than a decade, but they was sounding good when I replaced them. They have a guitar amp output transformer with 5K impedance for which I modified the power supply and the operating point of the tubes. The last time I actually wrote down anything on upgrading the amp was in 1997, it's in the October VALVE. They are not exactly detailed instructions! In that article you will see the first time I suggested converting the monoblock to stereo; that was the genesis for the current SEX amps, first introduced nearly a decade later.
