All three will help!
I used Buffalo phenolic faced plywood on my old Springer, not cheap but being dense it does soak up the noise,
Next is to search the scrappies or Ebay for a remote air filter odd a small dumper or similar, to help with the intake noise.
Third fit a small raw water pump, with a strainer on the inlet and use it to cool the exhaust before passing it through an anti syphon box, pass it through the hull above water. Use rubber exhaust pipe and don't let it run dry.
The biggest problem is noise transmitted through the steel hull, if your engine is 'hard mounted, ie not on flexible mounts, then there's little you can do about that other than pull it out and start again, assuming there;s enough space underneath and around the engine and gear box. if it's on flexibles check with a few local boat yards and see if there's a softer mount available, the old ones may have 'age / work' hardened over the years.
Cheers Stef