You are a lucky guy, that engine looks like it is new.
I would not go with anything smaller than a 12x8. But the landing speed might be a little too fast for that pitch of a prop on your plane. On my low wing plane, as pictured, I tend to land it fairly hot and fast anyway. That may not work in your case with your plane. If possible a 13x6 or 13x7 might work better. But with a lower pitch prop the engine RPMs might go to high in the air when you are flying around as the prop unloads more up in the air.
I had recently replaced the bearings in my old Saito FA-80 a while back and made the video below. It was sort of a cheap thrill to actually get the camshaft back in correctly on the first try. I had bought this engine way back around 1990 and I have been running it ever since.
I used a APC 12x8 prop as that was what fits my old low wing sport pattern plane to clear the ground with. The prop doesn't clear by much either. I ran Omega 5% nitromethane fuel which uses a blend of synthetic and castor oil.
I was getting around 10,600 rpms on the test stand with it.
My engine has a fairly high compression ratio. I had read that the early versions had high compression ratios. So you want to be careful with the fuel if yours is like this as it doesn't like a lot of nitromethane in the fuel.