When you say "clunk" do you mean it makes a "big solid thump" as it engages, or do you get a kind of rattling "thud-thud -THUD "CLUNK" as it engages?
I'm thinking that as it's had a mostly rescue boat life, the engines will have performed a lot more engagements of the grears than, say a cruising or fishing boat.
As OBT says, check your linkages. If it's a big solid over loud "thud" you're getting, check the engines' idle speed (and clean / re- lube your throttle linkages). IA single loud thud would imply too high an idle speed.
Multiple thuds before full engagement is as OBT says, gear linkage slack or your dog clutches' teeth are worn, in which case you;ll need the whole set of gears, otherwise you'll have new teeth on the clutch & worn on the gears.