She's a
BEAUT Rusty!
Pontirag has a good game plan, but I would substitute one quart of ATF for the motor oil at the first change, and go a little while on that. IMO you will be able to get the engine internals good and clean without multipule changes, if you are on a budget.
I seriously don't believe you will dislodge enough gunk to clog the screen, if you clean out the pan and the tappet covers beforehand.
If the inside of the lines have sludge
that bad in them, then the motor is more than likely going to need an overhaul anyway. That is my opinion.