Only 13.9? Should be right at 14.1 when operating properly.
Anyway I've never done it, but sources around the web seem to indicate 270ohm-330ohm resistor will do the trick. Wattage requirement is pretty low, like .5w or so. I'd pay close attention to whether or not your setup is working correctly before buttoning everything up as you don't want a hot resistor buried in your dash in the event it's not behaving as expected. In other words, your charge lamp would normally only come on just during bulb check (key on/engine off) then extinguish after the alternator starts charging. BUT, if the alternator fails, in your new situation you wouldn't have the lamp glowing but the resistor getting hot next to whatever it's touching...(if my brain is working correctly after a 14+ hr shift, that is.)
Lots of discussion on this subject in various places but most vehicle charging systems (even bikes) operate pretty much the same way, though a lot of the posts are people subbing an LED for the original incandescent bulb, instead of omitting the light entirely. That brings a tiny bit more complexity as a separate resistor is needed to drop voltage to the LED to prevent burning it out.
Richard