1. Why? The J31/41 are good aircraft without jets.
2. Cost. It would be very expensive to convert them into jets, and would tend to have higher fuel burn.
3. Difficulty. This hasn't been done before, so a lot of time would be needed to carry out the conversion, then lots of test flight and recertification.
4. The aircraft wasn't designed for jets, the location of jets, the stresses caused by jets, the electronics and fuel control of jets or the speed the aircraft would have. Therefore, extensive modification on nearly all of the aircraft's systems would be needed in order to install turbofans. The J31/41 wasn't designed to cruise at 400kts, so it's not going to cruise significantly faster than it already is.
For a few 19-seaters, what's the point???
Your bone's got a little machine