Sure. First off, the stock T generator, properly rebuilt and adjusted with a fun projects (or equivalent) 12v regulator will charge a 12v battery. It puts out plenty of voltage and current for the job.
The starter is another story. Put 12v on it and it slams the bendix into the ring gear with a lot of extra force. While plenty of folks have made the case that "I've run my T that way with no issues for years", luck has clearly been on their side. Simple physics and electronics math (V=IR, and P=VI) tells you this this is a recipe for electrical and mechanical disaster over time. It not only puts a lot more stress on the bendix, it puts twice the current through the brushes, armature, wires, field coils, etc. The big losers being the armature commutator and brushes. The field coils can take it, the other components not so much. Sure it'll work for a while, but all additional stress on the listed components will shorten their service life significantly.
So, the solution is to rewire the starter field coils. Stock they are wired in parallel. The 12v conversion wires them in series. Using the equation P=VI,(power equals volts times current) you get half the current when you do that. Thus, when wired in series you essentially get the same stress on all components as if running 6v. This sounds easy, but as I've discovered doing a few of these conversions now, it's a royal pain in the a$$. A LARGE soldering iron, big wire, good insulators, silver based solder and oodles of patience are all necessary to do it right. You also have to get the polarity/magnetism of the poles right on each field coil or nothing works.
I've included a few shots below. The old, used, rewrapped coils ultimately shorted against the housing, but they proved to be a good training and learning ground as this conversion has been attempted by many folks. Some successful, others not so much. After doing this conversion several times now, I've since decided I won't do it unless I start with a new set of field coils. Even when rewrapping the old ones, there's just too much at stake with respect to shorts, so New field coils is the only way to go.
Steve Tomaso just got my last 12v conversation and I don't think he'd mind my quoting him "Starter rolled in yesterday - nice job, my friend ! Very nice job on the brass snout bushing - fits a Bendix drive like a high end leather glove !"
Jeff