What we're gonna do here, basically, is take the header of a pipe from a stock bike and weld it to the chamber or muffler from another bike. In this case, I'm just modifying a stock M56 exhaust to fit on the M48/56 hybrid motor that you can put together with the MopedFactory batavus cylinder shim. (Treats/1977). For this example I'm only using a stock M56 exhaust, the stock m48 muffler would work, but the M56 muffler works too, and I dont need to chop up 2 exhausts for no reason. This would be the same application you might want to use if you are going to put a 2hp cylinder on a magnum and keep the stock pipe.
If you want to use an aftermarket pipe, you will have to buy one. It always helps to get a pipe that mounts on the same side, in roughly the same place. When you are doing this sort of thing, you want the header length and diameter to be close to the same as the un-molested performance exhaust you are using. You also want to try to use the stock mounting bracket as much as possible because it is probably a lot stronger than whatever crazy crap you're going to make out of zip ties and hose clamps. So figure that out first.
Once you have your donor pipe in hand, and you've figured out how you're going to mount it, you can cut the header off the donor pipe. You'll want to have a pretty good idea at this point of how the two pipes are going to come together. If you can get your header on there and let both pipes clear that's great, if you cant, just cut them off so they have as much extra as possible. If you are trying to save the header off the donor pipe or the muffler off the stock pipe, think about that too. Sometimes you'll get lucky and have one pipe that can fit into the other one in some place, like a A35 tecno estoril which as a big section in the header... say you want to put that on an A55, you can cut off at the big section and weld the smaller A55 header into it or something like that.
If you can cut one pipe off in such a way that you leave a ton of overlap between the two, that's the best. You can then line them up with both pipes in place, holding them side to side of each other, and look along the curves to see where the two line up fairly close.
It should look like this when you're done:
Once you have a perfect mate, you can clean up the material for a good weld. I don't care how good of a welder you are, especially with this thin stuff, you wont get a good weld if it isn't clean. You've put in this much work so far, so do it right and get a nice result ya dummy!
At this point, if you have a welder, you can just weld it yourself right in place. If you don't have a welder you have a few options:
A) Mark the weld with lines across the weld, the lines will line it up when you take it to a shop to have them weld it, and you will know the orientation of the two pipes. Before you take off, make sure that when you hold them together and line the lines up, you can tell where it is supposed to be. If you are doing a slip fit where one tube goes inside the other, draw the mating line around the edge of the big tube on to the small tube, and some cross lines too.
B) Put a rubber clamp over the two and ride it to the shop and have them weld it in place.
C) Drive it to the shop in the back of your car or truck.
C is probably your best bet, as you are sure to get it right the first time. If you do A, you might want to have them tack weld it and take it back home to test fit before finish welding.
If you take it to a shop they will probably know to do this, but if they dont, ask them to.
If you weld it yourself:
First tack weld the pipe in place on the bike. Be careful not to light your bike on fire, i've seen it happen to a beautiful mint condition white Cimatti city bike and it was heartbreaking. The guy who did the welding was a friend, and the guy who owned the bike was a friend and it was like 'sorry i started your bike on fire dude' ... awkward.
So yeah, tack it, less chance of you starting your bike on fire. Use wet cardboard all over the place to keep sparks from going into the carb or whatever. Keep a fire extinguisher handy. Dont be stupid.
Now you can take it off your bike and its a lot easier to hold and manuever so you can seam weld it.
Looky that there! Batavus with an M56 cylinder on it. What'dya know. In this picture with stock encarwi and stock M56 pipe, she was going about 32-33 mph, with plenty of torque.