I haven't been able to find a detailed list of differences, but I expect the usual things apply. Carb / intake diameter, exhaust, etc.
It's worth noting that the Jawa promotional material does say that the 30 mph versions have higher compression, so they probably ran a thinner or no head gasket:
Unfortunately the 207 spare parts catalog doesn't detail which parts go to which speed versions, but the owner's manual does say that the higher speed version takes a larger main jet in the carb, which implies various changes for better airflow through the system.
Also, you can put a 210 cylinder on a 207; it has much better porting and they are still available (treatland or jawashop). (edited)