It looks like you are building a scrambler, based on the thread title of course, and not going full blown dirt bike like Jimmy the Hog as you stated, so my question would be why run a 21" front wheel?
That is in no way meant to pick on your choice, and I know that a 21" is better in many off road conditions, argueably all true off road conditions, but a scrambler is much more of a mix bike. That beatiful vintage iron head sporty in your inspiriation photo would have had 18" front and rear or possible 18" rear and 19" front I am not sure. But that would be a great look and a great fit for most uses on a bike with only 6" of suspension which is all you can get out of a sporty frame without a ton of mods like JTH had to do. I.E. swing arm lengthening, chain guides and tensioner rollers.
With that limited suspension a 19" front would usuably be plenty. This is coming from a big fan of 21" front wheels for dirt based bikes.
Your choice of course, and neither choice is wrong. I just the vintage look of that scrambler bike which usually have a more balance front/rear wheel combo package. Think of the new Trumpet scrambler or the MotoGuzzi scrambler bikes.
MG is not a true scrambler but in the family in my mind....thats an empty place
The Trumpet is a true scrambler in that vintage style