The muffler for a Beetle if quite specific to the model, becuase of the boxer/dual exhaust set up, so you won't find anything that will fit on another car aswel.
That said, what type of engine is it? If it's a 1300cc, just forget about it. Too slow to bother, if it's a 1600cc you might still have some fun with it. I would sugest getting some double weber carbs tough if you are serious about it, and in that case, locate a local specialist in Beetle parts. You'd be supriced what can be done with the car both for looks and performance wise. Of course for best performance an engine swap is teh way to go. Either a retuned modded Type 1 (beetle) engine or go with the option of a 2litre type 4 or even a Porsche engine.
If you are going to work on upgrading the engine you'll likely have to do some changes to the front of teh car. Beetles tend to "climb up" over their front wheels if driven hard into a turn. If you're beetle is a 1302 or 1303 you'll have less of a problem with it as it has better front end suspension as to the older models.
