If you get the part number off of the alternator, you can get one that has the same size pulley's.

I think Yanmars use a Hitachi alternator --- you can get others, but as Al point's out watch out for the controller/regulator. Unless you want to screw around with it, you'll want to get a seamless replacement.

You can probably do a Google (or similar) search.

Your engine, I believe: http://www.yanmar.com.au/marine/gm_series/gm_series.htm

http://www.ebasicpower.com/c/YAN-D-ALT
(Go down the page, and you'll see the "standard" systems -- but be careful, if someone has upgraded the controller/regulator...)
3GM 3cyl Diesel 84-80 w/2 Pin Regulator Hookup HI 35 12 LR135-105
3GMD 3cyl Diesel 82-On w/2 Pin Regulator Hookup HI 35 12 LR135-105

[I think the "standard" was a 35 Amp alternator... If you have a Heart Interface (or Xantrex, now) -- it may have come with a special controller. Did someone upgrade this?]