If you are on a budget, Emgo makes a number of aftermarket mufflers like a "Dunstall replica". I have used them on some of my Tonti bikes with good results. They are not the best out there, as far as chrome quality, etc.but for the $$, they are not bad at all. There were a couple of members here selling nice looking, oem mufflers from a Cal Vintage. Those may work for your SP also.
Rick D.