Ok. But basically if I swap all rhe electonic board it should work?

I would really have to see it to give you a definatlive answer. Gen 3 Systema electronics are not the ideal solution, a better solution would be Gen 4, but the electronics cost even with a VUK would exceed the cost of the A&K gun entirely.

It is a difficult choice you face.

Yes....I really dont know what to do..
I fear that im stuck with a gun that worth nothing. Thats why I think to modify it complet fcc electronics (mosfet, cpu board, selector board) and try to keep the stock motor.
I would loke to put 1700-2000 on a systema but it no where to happen.
350-500 $ of part is still cheaper
I just need a bit of help and guts.

Well, even FCC electronics would cost you $300. And this assumes that your motor is fine, and the gun will work fine after the new parts are installed.

here some picture and dimension of the ECU /bolt stop switch.
maybe it can help to know if aftermarket will fit.
http://imageshack.us/a/img593/3705/vqfe.jpg
http://imageshack.us/a/img10/7050/lfir.jpg
http://imageshack.us/a/img534/4521/srwq.jpg
http://imageshack.us/a/img43/9742/7iyj.jpg
http://imageshack.us/a/img837/1369/vrs9.jpg

Well, I can tell you right now that it is not compatible with Sytema Gen 3 electronics. They use a 3-wire cable to control the FETs. Gen 4 uses a 4-wire.

It is not the dimensions, it is the programming of that board, the voltge and polarity in an out and selector interface that will be the issue. Odds of compatibilty are not good, and nobody wants to blow up a $150 Systema FET board or an FCC for that matter trying.

By the looks of it you got the blue ones, they are a match for the CTW, the black ones are a direct match for Systema Gen3 and the purple are also based on Systema but slightly different. Brad is right no matter what you system you choose in the end it will run you more than teh cost of the whole gun and another issue you will face is the motor if your motor is teh exposed core version you might want to look at a new motor to go with the new electronics.

http://imageshack.us/a/img829/9946/1qh9.jpg
http://imageshack.us/a/img11/272/cx6d.jpg
http://imageshack.us/a/img341/2515/hz0h.jpg