depends on engine case and crankshaft stroke, basis of 50cc engine. A old Austrian Puch case design started at 70cc and a reduced bore version was made to get 49cc to be marketed in area where 50cc upper limit meant a legal market- but the original cylinders would still fit on the case easily and a ‘large’ 80cc cylinder was also bolt on. the 90cc cylinder needed minimal machining- basic file base of engine case for insert. Italian Minerelli basic case had 45cc to 75cc cylinders available, simple bolt ons. Couple Japanese cases from Suzi and Yamaha started as 75 or 80cc- the 50cc versions are reduced bore for legal/marketing reasons again and simple retrofits result in original design size- a couple steel instead of cast iron liner versions go to 99cc. Honda 50 cub engines have 55,65,70cc versions and again are large bore cylinders on same basic engine case- the stroke of crank is another 2 millimeters in couple large size version of slightly different engine family but crankshafts will interchange in engine case. Old Puchs with 80cc and 8 to 1 compression were known to put out about 6 to 8 hp, 22 mm carb or 24mm and racing exhaust. These were often Maxi 2 speed units that were classed as motorcycles in some countries, mopeds in others. The sneaky British that have no speed limit restriction for pre-1973 mopeds get a frame and engine case with paper history that meets loophole and hot rod them with ‘stock’ cylinders -as in part number is found in old catalog- and pass their MOT inspection.
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