mmmm . . . alt at 1500 rpm.
the ones with a built-in regulator are going to drive the field pretty hard to try and 'recover the start,' as it assumes it's still in a car.
I'm assuming that the engine is reasonably fresh, it's got good spark and a clean carb, airfilter, oil, yada, yada. Any tune-up or wear issue that the engine has will severely limit its ability to generate torque.
if you turn the alt faster, the torque requirement should go down. if you've ever had a loose belt on a car alternator, you know that it slips worse at low engine speeds, indicating that the belt is the torque limit, not the alternator output.
you don't say how fast the engine is turning. The torque curve of a typical small engine is actually pretty peaky. Most small engine manufacturers will have the torque curve on their website; my advice is to look at that information carefully, then govern the engine so it runs a bit faster than peak torque under a light load, so as you load it, it falls 'up' the torque curve, rather than down, off the peak.
otherwise, yes, limiting the current into the field will reduce your torque requirement. it will also reduce your output current and raise your 'cutin' speed some, so you'll have to run harder and longer to fill your battery bank.
the real durable answer is either a bigger engine or a smaller alt, though.