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Looks like there would only be room (on one plane) for a 3 cyl with mostly stock CG parts. Building a new cam ring would be easy enough (compared to building an engine from parts) but getting it to turn 1/2 crank speed would need a second set of gears - like maybe a planetary set?
If the stock cam drive was used, yes, 3 cylinders. But I strongly suspect the crank and rods couldn't be done that tight.
Cam gear is 75mm OD, crank gear 40mm OD. Centres are probably around 52mm apart. Stock cg250 stroke is 65mm. There are tapered small block Briggs vanguard cranks at 66mm. With another 40mm of effective rod length to make that happen, we could squeeze 7 cam gears in. Not sure we have room to get the offset crank gear in then, though. For such problems, I like to cut out paper discs and play with them. If we could arrange it so we used a set of Briggs crank cam gears to run the Honda gears, we'd have all our gears off the shelf.
A cam ring would be far more elegant. KHK has some stock ring gears that may be suitable. Not hardened, so would need careful design and roller followers.
Cam ring speed = 1/ (n-1)
Where n is the number of cylinders.
2/(n-1) (n-1)/2 lobes. That's for a reverse rotation cam ring. It's also possible to do it forward rotation at lower speed. 1/(2(n-1)) with n-1 lobes, I think.
Edit. Oopsie, wrong number of lobes for reverse rotation. Fixed it.
