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Alan Sheidler

Daniel is correct.  The limit in CC for a gearbox motor is 125.

If I recall correctly, an allowance is made for a larger single-speed clutch-type.  More than a decade ago I ran a Biland SA250 (4-stroke twin) on a test basis at a Pro Solo.  Way too slow off of the line to be competitive there, perhaps better at a regular autocross?

As far as the overbore goes, for Moto engines at least, it is not possible to do like a car engine, as they have the Nickasil coating in the cylinder.  So, +0.020″ (or 30, 0r 40) is not something which can be done without a subsequent replating.  The answer there is to recoat at the stock bore, if some repair is needed.  Sleeving (typically with iron, possibly an alloy steel) is also prohibited.

There are “Big Bore” kits which increase the CC’s significantly.  Those of course are illegal.