This is how I cleaned my exhaust without a blow torch, on a 78 Puch Maxi.
Remove exhaust, and take apart, remove the nut from back of exhaust.
Now yow should have 4 pieces.
I will call them the main exhaust (the piece that connects to head)
there is the exhaust baffle, and a little exhaust baffle tube, and the chrome exhaust pipe.
The main exhaust - I cleaned with a scrapper for the outside part, and a metal clothes hanger for the inside part. Use the hanger in all the holes of the main exhaust scrapping as much junk as you can from the inside of the exhaust.
The exhaust baffle and the tube should be soaked in kerosene. At least one night.
Then that should be scrapped and cleaned with the hanger also.
The chrome piece I cleaned with kerosene and newspapers for the inside and a brillo pad on the outside to remove the rust spots. Then I waxed chrome.
It is a messy job so use alot of newspapers.