Hardware stores have "U" clamps in a lot of diameters. Same priciple as an automotive muffler clamp, just smaller.
I'd go get one and install it at least temporarily until a suitable worm drive clamp can be had. If that joint leaks you'll run lean and hole a piston.
Read the jetting threads and get yours right ASAP.
You can cut an aluminum can into a strip to use as a shim around your header pipe if there's any slop. Watch for soot where the header joins the pipe and shim it if you see any.
Also, a little NevrSieze or good ol' Phillip's Milk of Magnesia around the headpipe before you insert it into the muffler tends to msake future removal a bit easier. The metal-to-metal connection tends to get locked together over time, unlike the stock pipe's mica gasket.
Rumor has it the Phillips makes it flow more freely