The stock carb has some parts missing, coolant lines clogged, rusted and the slider is very worn etc. When looking at prices to replace the missing and bad looking stuff, it costs $$$. I took it apart now and started cleaning it, I found someone nearby who is selling parts of his DT125R and has a carb will meet him later this week. If I can't find a decent stock carb, I will look for the Dellorto.

The current chinese PWK in it I wouldn't risk running as the build quality is the worst I have ever seen. It's not just visually not appealing, the materials are so poor even the jets snapped during cleaning, housing is missing a piece on top etc. So beware this junk guys! Brand is most probably "Motoforce" based on similar listings.

The exhaust should be this one: https://www.athena.eu/en-gb/lightweight-racing-complete-exhaust-kit-P20782.htm

And the ignition is from a French manufacturer, Devmoto. They set a DT125R mapping by default on the ignition. This ignition doesn't control the power valve servo by default, if I am correct the Zeeltronic unit has this feature. Devmoto states on the website that it's possible to connect the two if you send the servo and the iginition unit to them as some modifications are required on the servo for this.
Do you think it could be worth it?

Some great advice here, but I'll just chime in with some alternative idea for food for thought, but it doesn't negate the great advice we have here.

Many years ago, I went down the TZR Belgarda route, which effectively ran a DT125RE engine, just modified porting. As a result, I ended up using the carb from that bike, which was a Dellorto VHSA 32mm carb. These are probably easier to find as they're used heavily in the scooter community.

I've never had problems with that setup, but I am not convinced that the bike is that rapid (despite all the mods I've done), but maybe I just got used to it.

I'd be looking at repairing your original carb or trying to source a replacement, if your cylinder is slightly ported as you say it shouldn't make much difference. Back in the day two-stroke tuners like Stan Stephens etc. used to wring a lot more power out of these engines, very rarely suggested a carb swap unless going fo a full race, rebuild-every-few-hours spec and in a lot of cases even recommended retaining the stock jetting. If your stock carb is really scabby try soaking it in alloy wheel cleaner for 24hrs.

The OEM DTR carb also has a slightly angled float bowl which means it's horizontal with the carb installed, a useful feature due to Mikuni designing the TM28SS specifically for quite a narrow range of Yamaha bikes. It can be a bit expensive to get one in good condition but the time and aggro saved (your bike will just work) is well worth it. The biggest problem with the TM28SS is the air screw seizing but this can be prevented by a smear of copper grease on the threads when installing it in the carb body (it's also worth doing this on all the other carb screws which retain the float bowl, carb top etc), and always spraying the entire engine/carb/radiator area with WD40 after washing the bike (get the 5 litre size with free trigger spray from a motor factors).

I think your Athena exhaust is the same as the Tecno expansion chamber which I've run on stock carb/jetting with good results. Is your ignition a Zeeltronic? If so they're very helpful and will probably tell you which ignition map works best with the stock carb if you email them.