Exhaust Choices for 81 kz440 A LTD

The po had cut the 1981 kz440 header pipes right after the cross pipe to fit a pair of sito(?) slip on mufflers I want to replace the mufflers with new slip ons but I don't know what I need to worry about. Do I need a certain amount of back pressure for the engine to run right? My budget does not allow for buying a Mac $350 us system so I need to buy less expensive ones and i can't find the same model of mufflers I have now. The bike runs great, I can maintain 120 kph all day with no strain but the exhaust is rusted thru and getting worse. I really don't want to start re-jetting the carbs when everything but the exhaust on this bike is stock other than a paint job. Here in Ontario if your exhaust comes out any other place than the end of the pipe it is considered leaking.

Harley mufflers can typically be found for cheap. I've seen a number of bikes over the years with that type of muffler replacing rusted through stockers.

What about the jetting? if the pipe is too open will I lose power?