Have you considered making replacement cleats? This is not difficult given average woodworking skills.
If the original ones lasted 30 years then they have done as well as cleats made of most other materials.
Since they are possibly subject to shock loads the right type of wood may outperform metal.
As they are large cleats already it may be possible to build rather larger ones (If my 45 yr old engineering is right strength may increase as the 4th power of the dimension) though this may not be necessary.
Strength could also be improved by laminating them together rather than by making them from solid wood. Shape with a jigsaw, a rasp and sandpaper.
Durability could be improved by painting with multiple layers of epoxy
resing within the cure time of the previous coat and then sanding
smooth. A further coat of UV resistant Polyurethane varnish
(fumes are poisonous) and they are done.