Trying to help a buddy of mine out. He has a 1990 20' SeaRay bow rider
4.3LX 262 cid The balancer spun apart and he needs to replace it.
Anyone know what the standard replacement for this would be? The shaft is straight and key way is in good shape.
Having a hard time finding the recommended balancer to replace this with.
Also... is it simply a matter of slapping on a new balancer or does he need to be concerned about internal / external balancing and should this turn into a rebuild?

And useful suggestions would be appreciated. Thanks.

Mercruiser #11990 no longer available

Do your homework (diameter, application, etc), but this is listed as a replacement:

https://www.summitracing.com/parts/pio-872002/overview/

Oh, to answer other question, I have had a few OEM harmonic dampner's (5.0's, 5.7's ) break apart on stock motors. usually very old, like your friends.

Get the right removal and installation tools, and will be very easy to do.

any concerns with balancing? I see the old one has a couple larger holes drilled out for balancing, should those be duplicated as close as possible?

Do not duplicate anything just get the proper one for the engine. You may find a aftermarket one that will work like fluid damper. Little more money if you cant find a stock one.