What type of boat and how do you plan to run it? A compression and leakdown test will help determine the health of the motors. At 900 hours, I would think it is due for a valve job at a minimum. If you plan to run the boat hard, I would go ahead and pull the motors and freshen them. If you are just going to cruise around, then pending decent results from a mechanical inspection you could run them until the performance starts to drop off. I had a friend that ran a 5.0L motor to over 1,200 hours before he sold the boat. Motor still ran like a top and used no oil but had lost some power and lost a couple hundred RPM at WOT.