The OS service has been very generous with me. They sent me an OS 55 AX in exchange for the OS 50 SX. That was the 2nd OS 50 SX that died on the same problem: Worn out ball bearings within 6 months of use.

The OS 55 AX fits perfectly in the mounting pattern of the 50 SX. No alterations were necessary except those on the cowling allowing the increased size of the muffler.