Coincidentally, I just pulled the alternator off my gf's YSE12 today - will check the nameplate details tomorrow and post details here soon after ...
Just posting this in case some one comes looking for an answer in the future.
The alternator on the YSE12 has no nameplate, but it looks similar to a Hitachi LR135-31 (not quite exact, but same mounting points). It has the Hitachi symbol embossed on the rear journal housing. The YSE instruction manual says its 25A. The service manual says its 25A, 14V @ 5000rpm.
One other note is that the idler shaft (PTO - power takeoff shaft) that drives the alternator & water pump sheaves is rated at only 2-3hp at 3000rpm for the YSE12, which means about a 50A replacement alternator would be the max. (if you can spare that much horsepower ;-)
If anyone has the model number for the 25A, please post it up.