Syntech/Chroma Cult/KMXKen Ypparila 
This has long been the most popular MIDI retrofit for the Chroma. It is also the standard, its sysex format required for a lot of the software that supports the Chroma (see for instance editor/librarian software for Mac OS/Mac OS X and Windows). Originally developed by Syntech, the kit was later offered by Ken Ypparila, a former Syntech employee, after the company went out of business. It has since popularly been known as the "Chroma Cult" interface, reflecting the name of Ken's mid-1980s Chroma enthusiasts club; and sometimes KMX, his company name. The box has MIDI In and Out ports, and offers excellent integration with the Chroma: it is programmed using controls and switches on the instrument's front panel. Ken made another batch as late as 2001. The last remaining interfaces are available for US$100 from him at the address above. [April 2008]
Greg of Analog Synthesizer Service "was offering a Syntech clone for a while ... I still have two units left: one original ChromaCult & one clone. These are offered without a case, as I've found it preferable to mount them inside the Chroma (board mounts on the left wood panel; two holes for the MIDI jacks are mounted on the rear panel between the serial number sticker & the memory protect switch)." He can produce more at a cost of US$90 each (without case; $100 with case). [January 2008]
Thanks to Jack Colburne  and Christopher Now for the photos on this page.