I
have a 1985 19ft Renken Boat that has a OMC 3.8L V6 engin(GM 229 3.8L
V6). I need new valve covers for it since the old one are rusted
through on the sealing flange

I'm not sure , but I think the later year aluminum covers ont he automotive
engines will be a direct fit. ie, looks good, and no rust. Cheap? ought to get
them from any bone-yard salvage engines. GM made a blue-million of the things.
so that should be easy.

concerning your manifold though, I would thinkt hat would be a dealer item
only, unless OMC has discontinued them. Not sure about that.

good luck.

tim