These are kind of expensive, but if you are going to be on the hook for a while or keep the boat for a while, the upgrade might be worth it.
Perkins 4.108 Serpentine Belt Conversion Kit | Foley Engines
This was just a quick google
, so I am sure there are cheaper kits out there.
I have heard horror stories about bent cranks on the 4 cyl moter because of over tightening. Do your research
. I have twin 6.354 motors that I will be doing this upgrade on for the loop in about 4 years.