I believe the C90 was a different engine to that of the C50 & 70.
You could quite easily give the C50 a bit more poke by simply fitting a C70 barrel, piston, and head on it, and swapping the main jet out of the C70 carb into the C50 one.
We did this to my mate's Honda SS50 which used the same engine, but had a manual clutch instead of the centrifugal one and a 5 speed gearbox.
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