I have several .61 SF, and the parts are getting harder and harder to find. Rings and bearings are plentiful, but to replace a sleeve, piston, rod, crank or a case are next to impossible find. And if you did find them the cost would be more than a new engine. The SF swings a large prop and preferrs a RPM in the 11 K area.

I would recommend a new engine like the Jett, Navorossi, YS or even Evolution .61 NX.

It would depend on the type of pattern plane you are looking to build or have.

Frank