There's no need for exhaust backpressure on a stock engine with a muffler.. Too much backpressure can melt a piston but too little will do nothing.
The objective of a muffler is to kill sound. Unfortunately this requires the exhaust gases be bounced around and blocked so their energy is absorbed... and this causes resistance to flow (backpressure).
On a performance exhaust some amount of back pressure is needed.. pressure in the pipe encourages powerful sound waves and helps the pipe operate efficiently.. but even here too much backpressure is dangerous... more dangerous than on a low power, low rpm stock engine.
I think this myth about a lack of backpressure being bad came from something else.. Under certain circumstances a leak at the exhaut port can cause an engine to run lean as fresh air may be sucked into the engine and lean the air:fuel mixture. A lean mix can overheat the engine. But lots of 2-stroke engines run with no exhast system at all and run fine.
If you can't find the proper part to clamp the header pipe to the muffler, there are all sorts of mechanical clamps (plumbing and automotive) that might be substituted.. if possible (the muffler can still be installed and removed) have it welded. Or build a clamp that fits perfectly.