Stock exhaust timing on a 26cc is 164 degrees.
The four port cylinders won't fit the old G-23 two port crankcase as the transfer bowls won't line up correctly with the transfer ports. there is not enough meat on the case to grind it out to fit.
They run just fine on 89, CR is not that high.
props and rpm are 18 x 6 ~ 10, 16 x 8 ~ 12
7,000~8,500 rpm