Looks like he welded in a generic muffler. First I would hop under the car and make sure there is not a big exhaust leak. Run your hands around the joints with the car on but cold to fell for a leak. If it is just the muffler that is too free flowing you can get a stock replacement for $200 1995 BMW 318I 1.8L L4 Muffler | RockAuto . However be aware that you might be replacing it every few years. Every stock style aftermarket exhaust I have ever used rusted out within a couple of years and needed replacement again.