Hi TxB
To clean out the exhaust pipe thread, try cutting shallow slots down the length of the thread, in a pipe fitting of the correct size (with a fine blade in an angle grinder) so it resembles a tap.
It won't re cut but it'll clear out any crud and give you a good idea if the thread can be saved, at minimum cost, not perfect, but cheap!
I picked up the idea from an old '50's copy of The Motorcycle (or something similar) years back where it was recommended for cleaning spark plug threads (with an old plug)
Cheers Stef