I believe there are less than 1/2 a dozen "common" alternator
frames that are used by companies. I believe the alternators that Yanmars use are Hitachi. If you contact an aftermarket alternator
company with the criteria your alternator has, they can help you.
-How many mounting ears
-measurement between the ears.
-pulley diameter and single
or double sheave.
Most likely you will want close to the same amperage. If you go too large, the belt may tend to slip easier.