I did not do this, but have been looking into replacing the cruise on our '90 Ultra.
Most of the older cruise control units and parts are obsolete and hard to find unless you want to buy used.The upgrade kit mention previously is long gone, obsolete and rare these days.
The older bikes used a "REED SWITCH" on the back of the speedometer to provide the reference signal for the cruise control and volume level control, I think this is equal to the signal produced by the transmission speed sensor currently used for cruise.
I also think model year 2004 is the lastest year that Harley makes an independent cruise control in the latest Harley Accessory catalog. The newer CV carbs used a different type button for the actual cruise control hookup to the carburetor - however, they seem to be completely interchangeable as I swapped the control lever from my '90 with the one on a 2004? CV carb I purchased that had the newer style cruise button connector.
I think your best chance is to make sure you have a working reed switch setup on the back of the speedometer or switch to a newer style transmission case that allows the speed sensor, and purchase the Harley accessory cruise for model year 2004 -OR check the on the '03 unit you have to see if it is the same as the independent system they sell ( maybe a dealer will let you look at the kit to see what all is in it. It may be the independent system just has the control switches too, and you may not need them. You will also need the matching "button style" control lever for a new cv carb , the bikes with cruise also had special idle cables so you could turn off the cruise by closing the throttle.
Wiring is another story, I do not have much on that other than know you need to run a new circuit as the older servo motors did not draw as much current as the newer stepper motors . As far as I know there are no plug and play kits out there , you will need to do it all by yourself.
has some good useful information but I don't think they sell a kit for yours or mine.
I am led to believe the following transmission case will work with a '90 flhtcu
Once again, I did not do this yet but these are the things I am led to believe are true- if you do it, be kind and keep track of what you buy (part numbers etc) and did to make it work and post it to help others in the future.