Nobody has replied yet so I’ll bite:
I am fairly new to stock Honda, but I’ll hazard a guess that you have a 3 or four ring manifold. When you look at the cylindrical manifold sticking out of the engine, it should have grooves machined into it. These are grooves for o-rings, and you can find them fairly quickly through any of the usual kart parts warehouses online (SharkShifter, Fastech, Acceleration, etc…). They can be had for $1.00 or less each, or bulk packs for around $15-ish. Pay attention to what you are getting, as there is also an o-ring that is sandwiched between the manifold and the cylinder, and I believe it is a different size. At quick glance they look almost identical.
These are consumable of course, and you will want to replace them fairly often. The local kart shop here usually adds a bead of high temp RTV silicone (the orange stuff) around the o rings before slipping the exhaust over it. The seal is much improved this way, and it minimizes the gunk that can and will blow out of that slip fit joint. The downside is you will need to clean it off the manifold and pipe whenever you remove it/re-install.
Other than that it is straightforward. O-rings, sealer if you prefer, slip the pipe on and rest in in the cradle mount, put the springs back on, and tighten the clamps around the cradle and pipe.
2010 Spirit [Birel] SP32 // SRS CR125 - Stock Honda [#08]
Hill Country Kart Club - New Braunfels, Texas - http://daledaugherty.wordpress.com/