I suspect that with the boat now 27-years-old, finding an OEM leaning post--if there actually was such a seating option offered by Boston Whaler--will be very difficult.
Many Boston Whaler boats have been modified to use leaning posts in place of pilot seats or in place of the enormously popular reversible pilot seat that acts much like a leaning post. Leaning posts made by manufacturers such as BIRDSALL MARINE DESIGN have been used with success.
Have look for yourself at the products from BIRDSALL MARINE DESIGN at
https://marineproducts.net/rocket-launc ... ing-posts/Although you mention that height is an important dimension, you fail to give readers any actual dimension for the necessary height, and thus it will be difficult for anyone to give a recommendation for a specific product. If height is an important dimension in your choice of a leaning post, you should give a dimension for the minimum and maximum height you desire in a leaning post. This will give readers some basis to offer you more specific recommendations.
In choosing a leaning post, I would anticipate that the height of the seat portion would be also very important and would be a variable that would be chosen with regard to the stature of the helmsman that would be driving the boat. You should consider the height of a leaning post seat area as another important dimension.