an alternative is to fabricate a replacement from heavy wall (sch 80) steel
pipe and galvanise it. This will last almost as long as the proprietary replacements
Problem with fabricating most of the stainless steels is that the nasty mix of temperature, sulphur and chlorides is very corrosive, particularly to the welds. That is why OEM exhaust
mixers are typically cast.
Re valve clearances - I think not doing this is more bad habit than anything - I remember many moons ago when we checked the clearances on my father's old marinised Ford tractor diesel
all were out of spec and one adjusting screw broke off.
Dockhead - with your Porsche why not do what the Subaru crowd do and pull the whole engine
out to set the clearances!