Re: 21' Sea Ray 210 BR, 260hp 5.7 merc....Sunken treasure turned project
It may have been repowered or maybe just a replacement carb cover. There is a second location for the engine serial number riveted to the flywheel housing - starboard side, near the starter. So, you should be able to verify.
Looks like a closed cooling system ??? . . . nice.
It seems like you have got a good handle on things, and the fact that the engine runs is a bonus. Were you able to assess any internal damage, like rust in the cylinders, etc.? Compression test & cylinder leak test maybe?
In terms of replacing parts, make sure to use marine compliant parts for fuel, ignition and electrical systems (SEA J1171, etc) It is a safety thing.