Bending a Puch performance exhaust pipe to fit a Garelli (NOI) is relatively easy. You want to fill the pipe up with sand to help it keep its shape, so seal it up really well, you don't want scorching hot sand spilling out halfway through the job. You can use aluminum tape to plug up the ends, then pour fine sandbox sand in with a funnel, but be sure to knock it down so the sand packs in as tight as possible.
Now to get to bending: You'll need a cheap torch. Don't bother with propane, either get yourself a MAPP torch, or go home and use the propane torch to cook some hot dogs and try to bribe someone who owns a MAPP torch. When you start bending, only heat up one side of the bend, the "outside" or the side that is going to stretch out or however you want to look at it.
Come up with a strategy, I suggest starting with the horizontal lines. The first bend you want to work on is where the header meets the chamber, basically this need to be almost completely level, which seems wrong because you're bringing the mount up even more out of line with the engine exhaust port, but getting the header to clear the rear end of the engine case is usually the hardest part. Then hold the pipe under your bike to get a rough idea of how it needs to be moved. The big Puch bend for the horizontal exhaust mount needs straightened out to the Garelli angle, once you've made more minor adjustments, line up the exhaust studs and get an idea of how much it needs to bend to the right and left, you won't be able to make the mounting holes meet without the expansion chamber hitting your tire, but just try to get it as close as you can comfortably.
To actually mount the pipe, you'll need to widen the holes for the studs at the header, and in the back, use a 3 or 4 inch bolt. Use a locknut to mount the bolt sticking out from the Garelli exhaust bracket, then counter tighten two more nuts on either side of the pipe's mounting tab.
Since this process is hardly scientific, there is always a margin for error and therefore it is recommended that you double or even triple up on exhaust gaskets to ensure a good seal.