The K90 is an exceptionally good engine . I just got pistons and sleeves for mine from Anatoly Karpov. They use a slightly different large bearing than do the CMB engines. I do have many of these bearings in stock.
The rod needle bearings are the same as used by the large CMB engines.
The rotor spins on a bearing, not in a bushing. I have several of these bearings in stock.
The rods are steel and will be difficult to replace if you ever damage one.
If you can find a used K90 in decent condition, you will like it.
Al Hobbs