You could use a Metz 54MZ-3, 54MZ-4 or 54MZ-4i with the Metz SCA3602M4 foot. The foot is interchangeable so you can use the same flash on multiple camera brands, and the Metz SCA3602M4 foot is the one you'd need for your R1.
Most vendors sell this flash including the appropriate SCA module (the foot). But, it may be may be tough to find one with the foot needed by the R1 packaged that way (I didn't see any with a quick search of a couple of vendors). You see the flash sold without a foot, too (or with an iso standard foot). But, you'd still need to find someone that has that SCA3602M4 foot in stock and get it if you want a dedicated flash that's aware of your camera settings.
You could also use a non-dedicated auto thyristor flash with your camera (for example, a Sunpak 383 Super, or even a Metz with an iso standard foot). With that type of solution, you'd need to use manual exposure and set the camera and flash to match for ISO speed and aperture. Then, the sensor in the flash would measure the reflected light during the exposure, terminating the flash output when it sees enough light for the aperture and iso speed you've set. IOW, the flash would not be aware of any camera settings (and vice-versa), and the only communication would be to fire the flash, letting the flash control the exposure via reflected light seen.