Start googling Mercruiser 470 issues.
If your original voltage regulator still works, no need to change to an alternator.
However if the motor overheats as much as 10 degrees, that motor will eat the voltage regulator and spit the head gasket
Just throwing parts at it and not trouble shooting is taking $100 bills and setting them on fire.
Start with the basics.
Compression numbers, fuel pressure, timing
Drain the block
Take the hose from the transom shield and tee it off with the 2 hoses going to the manifolds. Pressure the block and heads to 15 psi of air. It should hold forever.
If it doesn't hold pressure, listen for the hissing
Without a leak down test, how do you know it's the head and not the rings?
No sense pulling the head and spending money until you know what the problem is.
You can borrow a leak down test kit from any FLAPS
you are correct, its the lifter cover gasket. however that would not cause rust streaks. only a cracked head from improper winterization would cause rust streaks on that side of the motor.
