use the 20cc dish diamonds with mag heads and tight quench (yields ~10:1) and a moderate cam (say, 230@.050) and you should be able to easily run on 89 octane. a little larger cam (say, 240-250@.050) and 87 octane should be doable.

of you're still concerned about detonation, take some sandpaper rolls and a die grinder and polish the combustion chambers, and put a slight round on the sharp edges on the top of the pistons. that'll help eliminate hot spots, and add a CC or 2 to the chamber volume.