Lift arms are not universal, but are based on a standard. Manufacturers tend to put a bit of their own twist to almost everything OEM. You may find similar arms with slight differences as to where the lift link assembly attaches, or perhaps multiple holes. The core concept of the 3 point hitch is that it is completely adjustable. This is the reason for the right lift link assembly being a screw.
Although it's nice to have a matching pair of lift arms, it isn't necessary. Just ensure the distance between centers remains pretty much the same. You can also purchase weld on ends to rebuild arms if the ball end split. Try to maintain the original geometry of your system and you'll be fine.
I have used many a tractor with mismatched lift arms. As long as they are the same center to center, slight variations won't hurt anything.
Tom
