21 hp bs gold alternator question

Hello everyone have a weird one for you I think I know what's wrong but cant find any verification so I will ask you all.
I bought a gt5000 craftsman mower that the guy put a newer motor in originally it had a kolher. Well the motor runs awesome till you engage pto then the amp gauge discharges so I assumed that meant pto was going out so I replaced it . same deal I know this motor only has a duel circuit alternator 2-4 amp I am guessing that's not enough to operate the pto and the tractor as it runs lousy when engaged if you fiddle with throttle you can use it but its a pain in the you know what. There is 19 volts at connector while running I think I need to put bigger alternator in but can find nothing that says that's possible. or should I count my losses already spent more than I cared to on it. The motor is a briggs and stratton model 331877-0869-g5

its a 48 inch deck and it cuts great doesn't even bog down really I cut 4 ft weeds with it no problem just not running right all checks are where they should be original motor was kolher 20 hp with a 15 amp charging circuit when engaged I have 11.8 volts at pto and battery just low enough to cause my problem  question is can I replace the alt with a 16 amp one from what I can gather the flywheels have different size magnets but I have found some contradictory statements on that any idea don't want to get into changing flywheels they cost more than the tractors worth

If the coil sees battery voltage it won't last long!