I jusat can't remember exactly what bore & stroke did this engine have, but I remember its diameter was slightly bigger than its stroke, each around 3cm.
Blackbaron, I have to disagree with you. Four stroke engines are nearly half as powerful as 2-strokes for the same size (as an example, a 500cm^3 2-stroke GP bike used to have nearly the same output as the newer, 4-stroke, 990cc machines), and I guess for model engines this ratio could even be a bit worse than that.
However, they have enough power for a lot of applications. Unless weight is critical (such as in a 3D plane, or a heli), I believe that you can use a 4 stroke. If you don't need pure power, but enjoy swinging a larger prop, or hearing a realistic sound, or a smoother and quieter running, then you can use those "slugs".
Propnut, I'd really like to see some pics or video footage of them... My jap engine was not made to fly, but in case I make it run again, I have planned some wicked ideas.