if the only problem you face is the connection between the header pipe and cat pipe is rusted out find yourself in your town a car repair shop that does welding and will do or does custom exhaust work. they will have the know how to do this.

look for a shop where the owner or workers work on stock cars because they know how to tweak a car.

what you want to do is quite simple.

1. reciproacting saw off the short length of pipe coming out of the front of the cat
2. reciprocating saw off the last inch or so of the header pipe that has the spin flange and the swelled pipe end
3. have the shop make a new length of the pipe for the front of the cat pipe to the header pipe and weld it to the cat and then either:

A. weld the other end directly to the header pipe

or

B. flare the end of the new length of pipe so it can be slid over the outside dia of the header pipe and then clamp it.

the cost of materials will be NILL
the time it takes to saw off the two pieces takes 5minutes total or less
and only the welding time which for good mig welder is about 10-20 minutes depending on whether or not he has to flare one end.

don't waste you money buying any new header or cat pipe unless they are bad beyond the joint where they meet.

i doubt it will cost 50 dollars if that to do what i have suggested.
if you can weld it will cost you near nothing

should you ever have to replace either section again for rot you can just cut off with the saw and start again.