ORIGINAL: Don M.
I made a home made header pipe out of copper elbows held together with JB Weld. It held up fine for the 100 or so flights I put on it. This was on a 26cc gasser, hotter than a 4 - stroke and the first joint was within 2" of the exhaust port in the cly. It will hold no problem if your prep is right.
I believe you will find that a four stroke's exhaust temp is hotter than a two stroke-----not cooler.
I have tried JB weld with less than satisfactory results on four stroke (unstressed) exhausts.