Well, let me say a few words about your steel fuel tanks. Steel can last forever if it stays dry. So keeping it dry is the way to avoid future problems.
Often water gets on and under steel fuel tanks from the fuel fill fitting on deck leaking around its base, down the fuel hose and onward from there. So the first thing I would do is to rebed the fuel fill fitting.
The other thing, but there probably isn't much you can do about it, is to support the tank on stringers so there is no basin for it to sit in, get wet and rust out the tank. Unfortunately most older boats just layed down a layer of foam and dropped the tank on it during building. Other than totally pulling out the tank, there isn't much you can do about it.
So keep an eye on the bottom of the tank. If it ever looks like it is sitting in water, do something about it if you can.
Lehmans are great engines. One thing to be aware of, depending on the model of the fuel injection pump, is that there is an oil reservoir on the pump that needs to be refilled every 100 hours or so.
Other than that, keep it in clean oil and clean fuel, run it at a reasonable load and it can last almost forever.