wire wheel as much surface rust off as you can, coat it with oil.
start haunting junk/antique stores for a saw vice, raker gauge and tooth set tools, sometimes called a spider set gauge. look for good quality files, knife edge fine cut with the back side smooth (or safe edged) for getting into the valleys, and a good fine bastard mill file. (yes its really a bastard mill file lol)
Theres a bunch of videos on the Youtube about it.
In a nut shell...
clean the saw, remove the handles
straighten any kinks as best you can
section by section level the teeth so they are all the same length (the old timey saw shops or filing shops would have large saw vices that could hold the entire saw in one solid fixture) you will need to shift the saw over in the vice so its supported for all your filing. (it can be done free hand... but it really isn't fun at all)
File the pointy teeth "cutters" evenly so the come to a sharp point
file the rakers until a sharp point,
on the cutters and rakers try to not file them shorter then you did after dressing it flat (use a sharpie or marker to highlight the very top stop filing as soon as it goes shiny)
Using a set gauge or set tool, tweak the cutters outwards, so the flat side sticks out a little bit from the body of the saw (usually around 1/32") on each side (this opens the kerf and allows easy movement of the saw)
then using a small hammer tap the rakers until they bend slightly away from themselves, and become just a bit shorter then the cutters (this sets the depth of cut)
reattach handles.
Keep some kerosene or thinned bar oil on hand and keep the saw lubed in a cut.
If done correctly is will keep up with a stock chainsaw... in the right hands... it will also give you a hernia, make you hate life and everything in it, you will consume more water then you ever thought possible...