how many watt stereo with 60 amp alternator?

It all really depends on how much power your Kenwood amplifier draws. If it draws 1000 watts RMS then your poor alternator won't be all that happy. If it is 500 watts RMS or less then stock alternator will be ok. Upgrading factory grounds will help quite a bit as well as beefing up your cable from alternator to battery.

If your amplifier is a class A/B then that is something that can cause problems if the amp is bridged. If it is a mono amp class D then it'll be nice and efficient.

Lots of factors involved!