2012 explorer engine

Don't attempt it, even if it bolts up, which I doubt. Today's engines can be described as a computer with an engine attached, not the other way around. Although anything is possible if you have enough cash, consider. In 1999 you could have had a 4.0 OHV engine or a 4.0 SOHC engine. If memory serves there were about nine additional pinouts into the PCM with the 4.0 SOHC engine. You are going to have a lot more sensors in the 2012 engine and may have to change out the PCM, plus additional wiring to concern yourself. Then you'll have to determine if the PCM will be able to talk to the 99 transmission to make it shift properly.