Ok, I had a lot of fun and managed to install mosfet, its a blast, trigger response become much quicker, plus no more problems with contacts ever. Did not take a lot of skills to install it, if you know electronics and have some soldering skill.... and a luck... and ~4 hours....
But after I had assembled it back, got some problems:
1. Feeding problems about ~30% shots.
2. Single fire cutoff does not work in ~50%
Fortunately Red62 offered help, he disassembled gearbox and pointed where I screwed up,
it took an hour, and now gun shots like new! Thanks again!