Then simple things first.
slight extension of the exhaust away from the cockpit with perhaps a diffuser.
better rubber mounts to stop vibration getting from the motor into the airframe. Check that there is no place the engine touches the airframe.
Check Prop balance
Change the Prop pitch to lower RPM at cruise...not best for the engine though !
beefed up sound adsorption behind the firewall...there are many commercially available cloths, barriers etc. Quite often there is plenty of space for a thick absorptive material Check that there are no holes in the firewall.
better carpet in the cockpit to adsorb the noise
look for anything that vibrates etc.
Naturally composite aircraft are generally much quieter
