The original sounds a lot like Luc Besson's The Professional. The pic was released in 1972 and starred Bronson as a professional hitman who is planning to retire and has found young apprentice (Jan-Michael Vincent), but there is some confusion as to what the young man's overall goals are. Stallone would, of course, be taking on the Bronson role. There is no word on which one of the young actors mentioned above is in the running for the role of the apprentice.
Pic is set up at MGM and said to be budgeted at $40 million.
IMDb has a remake of The Mechanic listed with a screenplay penned by Shane Salerno (AVP-R). There is no director attached and no mention of Stallone and based on the plot that says it has been "reimagined as a spy thriller" it doesn't sound like the two projects are related.
Source via Ryan Gosling Online