The '87-spec cluster with the electronic speedo causes some sort of minor short when installed in the '81 dash that's in my GT … so having gone to all the trouble of getting rid of the drive-cable by putting a sensor on the gearbox itself, I went looking for a way to use my '81 cluster with mechanical speedo along with the sensor on the 'box.
The only thing I could find, and I don't know if it's noisy or not, was a motor to attach to your speedo, and which varies speed with the frequency of the input signal. They weren't cheap either, I can't remember but I've got something in the order of ~$400 in mind, and then you had to work-out how to install it.
It might be more reasonable to see if you can find an instrument specialist who can swap a new generic mechanism into your R-Sport speedo? Actually it was an instrument specialist who said the aforementioned drive-motor exists, so you might need to get in touch with one anyway … I'd hope you can find something closer to home, but if not, it was these guys not that far from me: https://www.manlyautoinstrumentrepairs.com/ (coupla years ago now). Thinking on it further, might be worth pulling the speedo out & seeing whether the mechanicals will separate from the dial itself - can't see why it couldn't be a DIY job, assuming the replacement mechanism is configurable/adjustable ...