What would be the impact of replacing the engines on the 737 MAX, not just reworking MCAS? There has obviously been a lot of focus on the MCAS failure following from the 737 MAX crashes. However, the MCAS appears to be a compensating control for design decisions, specifically the replacement of the CFM56-7BE engines on 737NG with the much larger, more efficient LEAP-1B. That in turn necessitated a higher ground clearance. 

In lieu of redesigning the landing gear to provide greater clearance (and raising the overall height of the plane), the engine pylon was redesigned, moving the engine further ahead of the wing and closer in line with the wing surface. There were other changes as well. 

The re-positioned engines changed the thrust line. The resulting control characteristics were compensated for in software, with the MCAS addressing a very specific negative flight characteristic.

Notwithstanding the new engines being more efficient (resulting in lower operating costs), is it possible and what would be the impact of retrofitting the existing fleet of 737 MAX with an engine and pylon in the previous position ? Could for example, the CFM56-7BE and pylon be retrofitted?

There are plenty of plane models offered with multiple engine configurations, though I imagine they are fit off a similar pylon / housing.

If I were an operator of MAX fleet in which the public has lost confidence in flying on, I'd like some other option than scrapping the fleet. Edit/Addenda: I don't know anyone is planning to scrap the fleet and it's far too early to suggest what the final outcome may be as this story plays out. There have definitely been order cancellations, orders lost and questions surrounding the plane's certification. You can't cancel an already delivered/completed order; it would be difficult to force Boeing to buy them back and replace them (with what?). IF Boeing offered to "retrofit the MAX as an NG-like", it might be the best possible outcome for all. 

It certainly seems less drastic than Boeing starting from a completely different base design or losing customers to Airbus.
Possible? Yes.  Economically feasible, not even close.  To re-engine the plane, you would basically have to re-do a large amount of the FAR 25 certification.  Right now, Boeing has to re-do one thing, and it will take a few months.  To re-engine the plane would take several years at the least.  However much money it is losing right now, it is a drop in the bucket compared to the cost of what you propose.