Don't be afraid of working on tensioners. Tools for Self Reliance has an excellent repair manual/tutorial for rebuilding old sewing machines. Tensioners are one of the things they cover. Take pictures as you disassemble things and keep parts groups/ sub-assemblies together as you go. This will help you come time to put things back together.
You'll also get all the advice you need from here.
Your machine looks pretty bad now but I think you'll be surprised at how well it will clean back up.
Rodney