The two main possibilities to make the new '600GP' class affordable, both of which seem likely to cause some form of conflict with FGSport, are:
1) To use modified production engines - a clear breach of FGSport's 'not derived from production machines' statement - but why else would the MSMA have chosen 600cc four-cylinders?
2) For a prototype 600cc four-cylinder engine to be designed and built by an 'outside' manufacturer and then made available to all teams in the class. But who would build the engines, could such a machine effectively promote different brands - and can you really have semi-mass-produced 'prototype' engines?
What seems certain is that the 600cc chassis will be a pure prototype, although as the WCM MotoGP team found to its cost in 2003, if an engine contains certain production-based parts - whether housed in a prototype chassis or not - the motorcycle is not considered a prototype.
Meanwhile, Dorna has hinted that should 'problems' arise concerning the definition of prototype machinery for the 250GP replacement class, it may start asking questions about possible 'prototype parts' present in the World Superbike Championship...
The 250cc World Championship began in 1949.