AIUI
Different and more powerful engine equals:-
different prop, equals different gyroscopic effects, equals different weight and balance equals increased fuel flow equals re-doing the fuel flow calculations, equals rethinking and testing the cooling arrangements, equals new baffles equals changes to the cowling design equals a new test program to deal with the changes in performance equals re-stressing of the engine mount (even if it fits), equals rewriting the POH equals trebles all round for the regulators who find they have lots of lucrative chargeable work to do.
Of course if someone's already jumped through all the hoops before it's a lot easier.
Relatively minor changes are much simpler. Some of the small Continentals can be upgraded by component changes and a change from say a C85 to an O-200 is not too difficult because the engines are very similar. However there can be major issues if for example the engine has to be shifted forward because there is insufficient clearance for the new paraphenalia sticking out of the accessory case. (In the Luscombe they got round it by fitting a dishpan in the firewall rather than a new mount).
(Are you asking in connection with the shiny new steed I saw you with at N Weald last year?)
Mike