I don't see an immediate connection between the starter rattling, and it making the engine hard to turn over, but it is possible of course that it causes both problems. You can find out simply by removing the starter and seeing if the engine is easy to rotate by turning the flywheel.
There is a lot to be learned by dismantling an old engine that is probably beyond its use-by date. However you learn much more if you do it carefully, one step at a time, and review what you learn at each step. We are here to help if you choose to do that.
