So an alternator will only push out as many amps as it takes to raise the system to the target voltage. So if your current alternator is putting out 5amps to keep the system at ~13.0 or whatever voltage it is hitting, the new alternator would also only put out 5amps to hit the same voltage. What a larger alternator does is allow you to use more electrical power before discharging the battery, while the engine is running.
So, do you have any electrical demands you are adding to the tractor or planning to add?
Most automotive alternators have an RPM limit of around 13000rpm. Since car engines usually only spin to less than 7000, they can get away with an 'overdriven' pulley ratio of something like 0.5:1, or alt is usually spinning twice as fast as the crank. Car alternators usually can't make full output until the engine is above idle, up to ~1500-1800 rpm, which might equate to 3000 at the alternator. On a tractor since the diesel engine will only rev to ~3000rpm, you can get away with a much bigger overdrive ratio and spin the alt 3-4 times faster than the crank. But, to do so you must make the crank pulley larger, or the alternator pulley smaller. On a V-belt pulley there is a limit to how small you can go because the belt itself does like to bend shorter than a certain radius, so more than likely you would need to install a larger crank pulley ('on top of' the existing one) if you wanted to make full output from the alternator without needing to rev the diesel up near max rpm to get it.
If your existing crank pulley and the stock pulley on the alternator you get works out to be about the same as what would be on a car, you could leave it alone and just rev the engine up to ~1500+ anytime you wanted to be able to use that max output of the alternator. More rpm is better because the alternator is fan-cooled and the faster it spins the cooler it runs, at least in the lower rpm range we are talking about. Often when people want an upgraded alternator they also end up 'smoking' it because they pulled a lot of current at too low of an rpm, overheating the alt. In those cases it may have been better to just build a 'fast idle' system (a tractor will already do that just by moving throttle lever) than to upgrade the alt.
Just things to consider.
