As usual these type of engine questions get answers that fall into to groups: Those who say don't do anything till something breaks and those who believe in Preventive Maintenance
By asking the question about hours it seems the OP is interested in hours in some fashion. Unless an engine is one of the newer types that have computer control modules that keep track of such things as hours and other operating info the only way to know hours is with a Hobbs meter. They are super cheap
and easy to install and I recently saw one advertised that didn't even need any wiring
. Apparently just sensed the power running through the already installed engine wiring
to increment an LCD meter. Didn't study it as my boat has meters on it.
My boat was last re-powered with used engines in 1998 which came with meters that showed ~2000 hours - but who knows what they were exactly.But I do know how many hours I have put on them each year and that is all I care about at this point.
My initial pre-install routine was to replace the circulating water
pump, have the injectors cleaned and serviced, degrease the engines, repaint them with epoxy/urethane, replace all rubber hoses, change fluids and filters.
Even doing these things don't guarantee no troubles but if things are routinely services it avoids making repairs
more difficult when they need to be done.
A friend of mine owns/operates a marine
and repair business and you won't believe some of the stories about having to work on sailboat engines that have never been serviced that are all rusted up and it really adds to the cost of repair from the additional time it takes to work on engines like this to just get things apart.
Scheduled maintenance by definition supposes you know the interval involved. Without an hour meter it's just guess work.