folks refer to those as the "UPS" magnum for the UPS dark brown and gold colorway. They've got the Puch ZA50 motor, which is slightly less common and more complicated than the more typical e50 single speed, but for most purposes it's a great motor. Knowing the engine though will help when searching parts, as some magnums came with the e50, so for example a "magnum" clutch wouldnt necessarily get you the right stuff whereas a za50 clutch would. the topend cylinders and exhausts and carbs etc are all mostly interchangeable tho. The exhaust port angle IS different for magnums though so in that case you do want a "magnum" exhaust.
For performance, a first step usually is just a performance pipe - As mentioned it needs to be a magnum pipe with angle port. Personally I'd recommend the MLM people's pipe here, but you've got options:
That should pair well with everything else being stock including intake and carb, you'll need to get a bigger jet but everything else should be smooth. this is a very reliable and satisfying setup.
The additional gains with a bigger carb and intake are small unless you plan to open up and port the cylinder, or move to a bigger cylinder kit, personally I probably wouldnt. If you DO go with a kit, ZA's have a bushing crank which is not well suited to hgh RPM. Ideally rebuilding it and putting a roller bearing crank is the best option, but i'm sure Ken Roff will pop in here with some advice too and he definitely knows his way around a ZA magnum.
Alternately you can usually do quite well with a milder piston port kit, like the metra 65cc or the generic "70" party packs on treats.
a final note, flipping the clutch! It's been mentioned above and one of my favorite mods in all of mopeds, it's free and easy and gives you a very nice "semi automatic" control of the shifting, simply let off the throttle and roll it back on and it shifts to 2nd. this lets you delay the shifting and get much better acceleration as a result. it's pretty great.