Scrapping KRUS would be a start, as would heavily taxing under-used land and second homes.
I wouldn't even scrap KRUS, I'd just make it that it only applied to direct farming activities with no other possibility of using it for non-farming activities. If you indulge in any other work, you have to pay ZUS, end of story. With land, definitely, and even more so with second homes.
At the end of the day I am sure there are other ways to collect the lost revenue
Are there? PiS are trying to figure it out and they can't. It's about 10 billion a year, which right now is around 20% of the yearly shortfall in ZUS. You could increase ZUS contributions, but that's a guaranteed way to enrage the electorate.
Raise the employees ZUS a couple of percent so that in zloty terms they pay a bit more
I'd actually like to see employers ZUS abolished and replaced with a system in that employees are solely responsible for paying. Self-employed people could pay on the same basis, which means that everyone is on an equal playing field - but I wonder how many people would be content with paying roughly 30% of their gross earnings to ZUS. Either way, voters would get a hell of a shock when they realise the true cost of their employment to the employer.
What surprises me is that people still believe that the money in ZUS is 'theirs'.