my dad has a '84 Imperial VC-230 with a 3.7L "488" inline 4.. I know it shares the cylinder head with a 460 Ford. but not much other info about it... water pump went on the lake one day and it got hot.. my dad brought the boat in on the kicker motor, but the big motor still ran fine. later replaced the WP.. took it out and it's as what he describes as a rattly knocking.. says it sounds like it's in the head but sounds like a spun main. also told me it sounds ok without much load on it.. kinda sounds like a valve rattle to me, but my dad said it all looks good and is tight.. any ideas?

what all will it take to put a SBC in this boat if the engine isn't worth fixing.. every resource and boat shop my dad talked to said to put in a 3.0L 181 4cyl as it's a bolt in, but this boat is close to 5,000 lbs. and stock 188 hp.. it gets on plane at about 25 mph and will run until 44 MPH or so ( depending on how rough the water is ) trimmed out all the way. I'm not sure 135 hp is going to even get it on plane