Actually the Rotator mid drive cassette is mounted on a pair of sealed bearings with an ID of 10mm. The shaft is a 10mm axle cantilevered off the main
tube and held by a small tube welded at a 90D angle to the main tube with an internal 10Mx1.0 thread. The cassette splined carrier is standard Shimano
splines and the sealed bearings are at either end of the splined carrier. Inboard of the mid-drive cassette is the drive cog for the rear wheel cassette.
All of these cogs can be obtained by looking for cassettes held together with screws and not using carriers for the larger cogs. These used to be available
for $5-10 from Nashbar but have gone up the past few years. The better known Shimano/Sram cassettes all use carriers for the larger cogs and are not
suitable cog donors. You need to rummage for off brand 7-8-9 speed cassettes without large cog carriers. Rotator used standard 10Mx1.0 solid axles for
their mid-drives and had to shim the middle of these with a wraparound shim glued to the shaft for them to fit well into the bearings. It only takes a few
degrees of skew in the axle to allow the rear chain to derail under torque.
The axles had an actual diameter closer to 9.5mm where the bearing fit and 9.8mm at the threads, at least on my axle. The skew angle problem necessitated
manufacture of a new axle at home. First attempt with carbon steel solved the skew angle problem but broke after 800 miles. Second axle out of 4140 lasted
5500miles, and the 3d axle also 4140 is still on the bike after 13,000 miles. I lengthened the axle so I could put a nut on the L side of the mounting tube to snug
up the axle and made wrench flats on it for easier removal 'if it breaks'. The axle is under
a lot more torque than you would think, as both breaks occurred with strong accelerations, fortunately not too far from home.

The Sunset looks more like an idler set up than a Rotator/Trek style middrive.