There is one thing that is pretty common on almost all scooter and even some larger bikes, the handlebars. In most all cases where a bar measures larger than 22.2mm/ 7/8”, from the center of the bar to the controls it will taper down to the 7/8”/22.2mm controls. This allows for most all grips and controls to work between many types of bikes. This also allows them to be used with many types of bar stems to hold on to the bars as well. Grips are also all universal, so you have choices in rubber to foam to add as much hand grip and/or comfort as you need or want. Some gauges are also universal, as reading head temperature, water temperature, RPMs, fuel level, and lambda (air/fuel ratio) readings are all standardized in many cases. It’s usually the sensors themselves than can be specialized to the gauges.
Lights and their controls can also be universal or be used between manufacturers, but some wiring may need to be done to make them work. There may also be some bracket fabrication required to make a light fit or work, too. There are also universal lights that only require a power and a ground to work. Flasher Relays are fairly universal, but even adapting them isn’t much of a challenge if you’re good at crimping wires together. A couple of spade terminals and connectors will even allow you to retain the OEM connector most of the time.
If there is a common theme in any universal part it is this: Adapt or Die! Even if a part is universal, there may be adapters, brackets, wiring, or even fabrication required to make a part work. Manufacturers try to plan for every possible bike for every situation they can think of. However, this is impossible considering most of these parts are sold all over the world for bikes of several manufacturers and even model changes between a single bike model in several countries. I would make it a kin to guessing the lottery in several countries and some of those countries may be 4 numbers, some maybe 6, some may use letters.
Universal parts aren’t always perfect either. You may have to trim them to fit, clean them up, or even paint them. Again, they are designed as universal parts, so some fit and finish is required to make them work. This is also the case where if you don’t know what you’re doing, it will be better to hire someone who does. If you can’t wire up lights, you can’t paint, or you have never turned a wrench in your life, this may not be the best time to learn; but even so, this isn’t always the case. If it is a part that requires you to tighten a bolt or a nut, yeah, anyone could do it. However, painting, cutting, and wiring are completely different animals. Those issues are things someone with experience should handle. So don’t be afraid of universal parts or having to stick with one aftermarket manufacturer, it’s all about making your bike yours!