Or do it electronically, depends how much the load current is but you can get some fairly cheap small electronic units to give you a constant 12v output regardless of engine speed once its generating.
If I owned a 4" Showmans I'd also look at including a small'ish battery to enable the lights to run for a short time without having to belt up, also would allow the lights to run while travelling. Once belted up this would then recharge the battery.
Once you have a stable power source it then allows all kinds of modern paraphernalia to be fitted, how about a showmans with USB phone charging ports, charge your phone while at the beer tent
