I would decide just how much it's worth it.....if you have to buy one it will be at least $30 and then you have to hope you don't have fitting issues. I did the opposite, put a DAO hammer in a DA Model 10 and although it worked, the hammer didn't fit 100% "right" and the trigger pull got a little gritty, probably from rubbing on the sideplate and the firing pin hangs up a little on the bushing when I dry fire it.
The hammer should be stock, it's likely the SA notch was just ground off and the hammer despurred. This isn't an NY-1 64, is it? If so these are rapidly becoming collectibles.
Also, I bought a DAO Ruger GP100 and dropped in a standard hammer and the trigger pull went to ****, developed about 3 "hitches" in the DA pull....... once those parts break in to each other, it's not always good to drop a part that has broken in to another gun in there and hope for the best.......I ended up having to swap the DAO hammer back in anyway.
If it's a PD trade in you got on the cheap it may be better to keep it DAO and enjoy it for what it is, and look around for a DA/SA 64 later on, it will give you an excuse to buy another one