Option 1 :make a flange to accept the steel ring and use a stock replacement resonator and muffler available from bmw.
Option 2 :Ansa does make resonator and muffler and you can also get the s/s/ tip to look stock direct fit .
Option 3 find cast arn manifold ,repair cracks and block off and use a stock down pipe.
Yes its the simple /flaky route to weld and make the exhaust system ,non serviceable by what you have.
Used to purchase new fronts from bmw ,today they are no longer available.
There used to be someone who you send him your old belts and he would sew in new nylon .
Maybe he can chime in in this conversation.
J
Looks to be nice ,no erosion on water passages.
Next is warpage ,thickness,, pressure test for weak casting and cracks.
Cylinder heads are not a simple chik chak seals rebuild like in the ancient days.