DuBro makes a weighted washer designed to go between the prop and nut, comes in 1/2 oz, and 1oz. sizes. I use the 1 oz underneath the adapter nut for my Dave Brown aluminum spinner, also on a .56 Saito. This is on a Rascal 40, which gained some tail weight after a crunch and rebuild.

As far as props for this engine go, I started out using a 12-5 MA K-series on my 40 Rascal. It would turn 9800-9900 on 15% fuel, but my airspeed wasn't quite what I wanted, so I tried a 12-6, it only dropped rpms about 150 and gave noticeably better airspeed and climb performance. I'm planning on putting this plane on floats next summer and want more thrust, so I've tried a 13-6 MA. The plane really hauls, now, not much faster than the 12-6, but a bunch more vertical performance, which tells me more thrust; the downside is that it eats the rpm, dropped it to about 8200 and slows throttle response a little, takes more power to spin the prop. My next step is to try a 12-5 or-6 three blade to address the thrust issue, and I'm also going to try 20 or even 25% nitro and see what happens with power. Personally, from all the flying I've done on the Rascal, I think the engine is happiest with a 12-5 or 12-6. Of course a different style prop could give different performance, such as a APC. I also think that the weighted washer has a small mass effect, even though it is close to the center, there is a small inertial component that slows the engine's response.