I used to change mine ( regularly!) on a KAD 300. They honest best source of info I found was the manual. It gave a schematic of what to do, how tight to tension it and so on. On a KAD 300 the belt itself was not the issue - it was the agro of removing all there other belts and flywheel to get to the belt - which is pretty much loosen tensioner, remove old, clean up, put new on and tension up again.
As above it's easy enough .I had a kad 300 boat once .
From memory there is an unusual size socket /ring spanner -13mm? Needed
Also the pulley wheels are base steel and badly corrode in the Vee .This abrades the belts = black dust disease .
So while your in -rub down the Vee in the pulleys and coat in waxoil .
This cured the black dust for me ,and poss extends the belt life .