I have try to figure out this problem, looking to rewiring the stator but even if I get success, I still need to replace the regulator, the wiring, and possibly the ignition switch since I don't think the OEM one can handle 25A. Of course you can design a auxiliary relay to handle that but it's a lot of work and I prefer K.I.S.S solution
Plug the heater direct on an extra or bigger battery and your good to go, My Optima "Redtop" I salvage from my Diesel Jeep can old a 25A discharge for 100 minutes, enough to power you heater for a few hours (thermostat control wise) and this cost me NADA!
PS: My Optima is 8 years old and start my tractor like the engine run to catch a deer! The new Optima are fab by Johnson Control and don't worth crap...Buy a Sear Platinum they are made by Odessey.
