Hello!
Being the Swedish distributor of MVVS engines I must agree on what Gizmo and Jim says. The MVVS mini pipes (there are two types available for the .40-.49 engines) are not suitable for beginners. Simple as that!
What I recommed for all beginners using .40-.49 engines is the the MVVS ultra quiet silencer (No 3245A). With this silencer you get very low noice levels as well as good power.

No lenghty running in times are needed as all MVVS engines copuld be run directly in the airplane, provided you run the engine a little rich the first 3-5 tanks.
Prop to use would be a 11x6-12x4 APC for best performance in a high winged trainer.