Possibly just need to replace the o ring that is on the adjustment screw....easy peasy, just unscrew it right out after noting how far it protrudes so you know how much to screw it in on refitting, then replace o ring.
Is it the screw that is just above the stop solenoid?
Get an assortment of metric o-rings from Autozone. Chances are you will find the right one in the kit. They'd likely be BNR (buna nitrile rubber) which will be ok for diesel. If you want to run bio-diesel you'd want Viton. If you have a local hydraulic shop, try them also.
Nice going! Can you post the link to the Czech site?