The OS .61 RF is a long stroke engine, and as such it has a smaller piston.
I don't know if there is anything that will fit. Check the "OS-the sad demise"
thread....there's an outfit that sells replacement parts that they manufacture,
and a Guy that replates the existing parts.

There are some original parts still available, but they are over priced, and still
the same ABN junk. [sm=thumbdown.gif]

Dave.