Check the 3 digit Manufacturers ID Code (MIC) in the HIN. The Coast Guard has a good MIC reference data base.

as for the wiring diagram, your never going to find a boat specific one. Merc based?? That will give you a head start. you have two main looms, one from each engine. a little digging and you can find the color codes for the wiring. This will cover the ignition and gauges Trim pumps generally run off a separate merc loom, That leaves you with trim tab wiring, nave lights, stereo and misc stuff. Some times after an old boat has had things added, removed , changed and 'fixed' it may be easier to just rip it all out and start over as all that's left is a rats nest. The engine looms and trim looms are avail. from merc and aftermarket, then run a heavy gauge power wire and ground wire to the dash, a couple bus bars and start getting creative. I really can be one of those things that's easier to redo than to figure out whats wrong or has been bubbatized.