1965 YM-1

Seejay,
I picked up a 66 yds3 250 a few years back. I have done some work to get her back on the road and have learned a little about these bikes. Let me know if you have any questions, I will try to help. I do have a few manuals.
You are already in good shape as the toughest thing to find are exhaust pipes, your look pretty good!
You should be able to find a seat, they pop up on ebay every once in a while and there are repro covers made too.

I have purchased parts from yds parts in UK, Alasdair is reproducing many of the small parts and is a good resource and a great guy. http://www.ydsparts.com/

I have also received lot's of help from the guys at the yds owners group forum, these guys know the yds inside and out!
https://groups.yahoo.com/neo/groups/yamahaydsownersgroup/info?yguid=162475011

My wife would confirm that they are a blast to ride.
Block Island, RI USA